In the wake of the Fort Hood jihad massacre, Muslim leaders in this country are preparing a fatwa on the permissibility of serving in the U.S. military. It is likely to differ from this one, but the answers from this site are on the whole more respected than anything that comes from Muslims in the West. Islam-QA is a very popular site for Muslims to get answers to questions they have, and the late Muhammad Uthaymeen is very respected as well -- even in the West.

"Military service in kaafir armies and working as a "chaplain" in those armies," from Islam QA (thanks to Axel):

What is the ruling on Muslims serving in the military of non muslim country ? What is the evidence for its permissibility or prohibition?

And lastly, what is the status of a Muslim working to help those in the military to fulfill their obligations to Allaah while serving in the Army, Navy, or whatever?

Praise be to Allaah.

We put this question to Shaykh Muhammad ibn Saalih al-'Uthaymeen, may Allaah preserve him, who answered as follows:

"Praise be to Allaah, the Lord of the Worlds. Military matters are problematic, because they involve helping these kuffaar to wage war against the Muslims or those who have entered into a treaty with the Muslims. If no such thing is involved, it may be advantageous for Muslims to work in these armies so as to learn their secrets and be aware of their potential evil. In other words, if working in these armies could be of benefit, it may be permissible, otherwise it is not allowed."

On this basis, if a person works as a preacher or daa'iyah or imaam or muezzin, serving the Muslims and calling non-Muslims to Islam, then there is nothing wrong with this.

Islam Q&A
Sheikh Muhammed Salih Al-Munajjid

Then there's this:

It is not permissible for a Muslim to fight with kaafirs against the Muslims at all

I work in the army of a non-Muslim state, and there are wars between them and the Muslims. What is the ruling if they send me with a division of this army to wage war against the Muslims? As a Muslim, my feelings are that I never want to fight against Muslims in any war.
What should I do?
What is the ruling if I go...?

Praise be to Allaah.

If you are sent to wage war against the Muslims, then it is not permissible for you to take part at all. Helping the kaafirs against the Muslims is a form of major kufr which puts one beyond the pale of Islam. Allaah says concerning one who supports the mushrikeen (interpretation of the meaning):

"And if any amongst you takes them (as Awliyaa', i.e., friends), then surely, he is one of them"[al-Maa'idah 5:51] [Koran 5:51]

With regard to how you may get out of this situation, and what excuse you can give to get out of this dilemma if it happens, we ask Allaah to help you, and we suggest that you consult some Muslims who have relevant knowledge or experience.

We want to emphasize to you the necessity of finding other employment and of leaving service in the army of the kaafirs, because that implies helping them, strengthening them and increasing the numbers of their fighters and supporters - unless your work can bring some benefits to the Muslims, such as giving information and secrets of the kaafirs to the Muslims so as to help the Muslims, or if your work is purely da'wah, such as giving khutbahs and leading prayers for the Muslims in the kaafir army whilst also advising them to avoid any work that will strengthen the kaafirs. We ask Allaah to keep you safe from temptation and to give you a good end in this world and in the Hereafter.

Islam Q&A
Sheikh Muhammed Salih Al-Munajjid

And this:

Working in the army of a country in which there are munkaraat (haraam deeds)

Please tell me about the ruling on one who works for the army, which is the source of his provision, but the military system and laws oblige him to shave his beard and some of them have to respect others, as the foreigners do. We have to greet others in a manner different from that enjoined upon us by Allaah and His Messenger, we have to salute the flag and we are governed by and have to refer differences amongst ourselves to laws other than those of Allaah (military law). If I fight to defend my country, but it is not under the banner of Laa ilaah ill-Allaah Muhammad Rasool Allaah [There is no god but Allaah, Muhammad is the Messenger of Allaah], and by the will of Allaah I am killed, what is the ruling on me according to the Qur'aan and Sunnah?
Can I fight with an intention that is different to that of the army in whose ranks I am fighting? If I do the things mentioned above so as to avoid any negative consequences, am I sinning by doing that? Can a Muslim work in the army with the intention of learning fighting skills which he cannot learn anywhere else under current circumstances?
Please tell me about obedience to parents in this matter, when points of view differ in a case where the parents do not follow the Qur'aan and Sunnah, but they follow the traditions of society and whatever people are agreed upon. They think that religion is just prayer and fasting, and that anything beyond that is extremism. May Allaah help you to do that which pleases Him, correct your mistakes and protect you.

Praise be to Allaah.

Firstly: it is haraam to shave the beard; it is obligatory (waajib) to leave it to grow.

Secondly: it is not permissible to salute the flag.

Thirdly: it is obligatory (waajib) to govern by the sharee'ah of Islam, and to refer to it for judgement. It is not permissible for a Muslim to greet colleagues or superiors with the greeting of the foreigners, because of the reports which state that it is forbidden to imitate them and because this represents exaggerated respect for them.

Fourthly: Whoever fights to make the word of Allaah supreme and to protect the Muslims and their lands from the enemy, is fighting for the sake of Allaah, and if he is killed he is a shaheed (martyr). What counts is the aim and intention. You can have an intention which is different from the intention of the army, such as intending to make the word of Allaah supreme in your jihaad (so long as it is permissible, according to sharee'ah, to fight the group against whom you are fighting), even if others have different intentions, such as fighting for their country.

Fifthly: it is obligatory (waajib) to obey your parents so long as this does not entail disobeying Allaah, because there should be no obedience to any created being if it involves disobedience of the Creator.

And Allaah is the source of strength. May Allaah bless our Prophet Muhammad and his family and companions, and grant them peace.

Fataawaa al-Lajnah al-Daa'imah, 12/22