I'm in the market for an HDTV to use as a monitor at the moment, but everytime I search for one I find that some 1080p TV's only output 720p computer resolution; some say this in their specs, others don't. I'm wondering if this is because manufacturers neglect to put it in sometimes, I don't want to purchase one that can't do 1080p. Does anyone have a model that they recommend which for sure can do 1080p outputted from a computer? Also, how is the picture quality/experience? Thanks. 

I'd suggest you be cautious and do a lot of research including on AVSForum.com when making a decision.  I've heard of HDTVs that have TONS of display lag when hooked to a PC, meaning you can even feel the lag moving the mouse cursor around.  I've personally not heard of 1080p LCDs that won't accept 1080p output from a PC at all though.
 
Probably not much of an issue, but make sure you can eliminate any overscan (I'm not sure how widespread factoring in overscan still is on HDTVs, but it used to be that a certain portion of the screen was cut off on all edges).  Not sure if that's what HDTV PC modes are used for or what.

What size man?
 
I can happily recommend Samsung because they don't fuck around with selling 1366x768 sets as 1080p capable just because their image processors can take 1080p input like some other less than honest manufacturers will.

They seem to be selling 1080p lcd monitors like there is no tommorrow. However since I used a dvi to hdmi cable before, it depends on the game, crysis and resident evil 5 for example wouldn't support the 1080 resolution of my hdtv, well my tv is 1080i and alot of the games just says 1080. However games like gta4 and everything else like half life 2 on my steam list does. It seems a bit of the image is cropped when playing these games though. You will enable 1080p in the game options, also if you want to see that with your desktop, you will add these supported hdtv resolutions in the nvidia control panels.
 
I think it would work tbh with a hdtv, if you do get it, blog about it, I really want to see the results in full 1080p, not just 1080, or 1080i which I used. Still games really looked quite good at 1080 settings.

I have a 42" LG, that runs at 1080p. Unfortunately this particular model doesnt play nice with my PC, so whenever it's hooked up I get lag issues and such.
 
I would definitely do the research before you purchase the tv. However it is totally worth it, as games on a flatscreen look UH-MAZING.

"What size man?
 
I can happily recommend Samsung because they don't fuck around with selling 1366x768 sets as 1080p capable just because their image processors can take 1080p input like some other less than honest manufacturers will. "

I have hooked up my Mac Mini to my 46" bravia and the computer immediately recognized my Tv and calibrated itself too and the thing played in 1080p like a Mo Fo. That was for HD movies running at 1080p 25-31 Mbps without a single hitch or lag but I guess you are talking about games and a windows based PC so I really can't help you there. 

No input lag here, then again my hdtv is a sony rear projection front lcd and I didn't notice any of the 1080i problems ppl were noticing. Here is a sample if you don't mind to show you that you can use a hdtv as a monitor no problem.

   
Dragon age orgins has a bit over overscanning, cropping up the icons there 
 

You can get rid of the overscan in the videocard control panel or disable overscan.If your tv has this option put it on "just scan".