1) $200k is pretty weak, even for today (it's gone downhill overall). If you can get a secret (basically, us citizen and not a felon), and have decent tech skills, you should be at $200-350k.
2) It's not tax free. The FIRST ~$85k is tax-exempt, which saves you about 18k. You then pay taxes at the normal 27-36% rates above that. My win s that I don't have an apartment back in the US, so my only expenses while here are car payment, storage, and server colo.
3) Big difference in quality of life throughout the country -- if you're on base at Balad or Victory it's basically like living in a small town in the US. If you're out in a Bn or smaller unit on an iraqi army base, or out in the city, it's a lot different. I've done a variety.
4) If you've never been in the military or had contact with it, I'd do it for a year just for the experience -- it's actually something I think everyone should do for a couple years.
The biggest downsides for me:
1) I gained about 20 pounds last year in Afghanistan living on Bagram, due to the shitty but high-calorie carb-laden food.
2) Generally, working for government is the anti-startup. You are specifically instructed not to take any initiative, everything is more concerned about process than results, etc. Actual combat is different, but there is no real combat in Iraq or Kuwait now, and in Afghanistan it is unlikely a contractor involved in direct combat support would be posting like this on hackernews. (If I could get a job where I got to put bullets into bad guys 12-24h/day, I'd do it for $20k/yr, but aside from joining a government agency or the military, it would be difficult to find a contractor in 2010 who does this ... blackwater/triple canopy/etc. etc back in 2003-2005 were about the only ones)
3) There is no firm end date. I did this for 6 months just to see what it would be like, and have wasted about 4 of the past 5 years in the sandbox. I'm planning to leave in a few months to work on a startup of my own in the USA again (maybe YC 2010 Summer!), using some of my experiences from here, but it's totally possible to keep doing this indefinitely.
4) The war is definitely winding down, so pay, upward mobility, etc. continue to decrease
5) People in the defense industry are NOT like startup people -- there are huge amounts of fraud, backstabbing, incompetence, etc. Startup deals for $50m can be done on a handshake; here you need lawyers, contracts, audits, etc. or you will get fucked.