There is absolutely no contest between a 1080p TV and a high res monitor. The only people that say otherwise are the ones that cant afford a decent monitor and/or no nothing.
I have a 37" 1080p Philips, and a 46" 1080p Sharp TV, they do 1:1 pixel mapping and have a higher colour gamut than most TV's - both high-end in the thousands, but they dont compare to my 24" Dells or other good monitors i've used. Theres so many reasons, but heres some of the more obvious/bigger issues...
1: While 1920x1080 is a pretty high res, it aint all that sharp on a 30+ inch screen compared to a monitor. It's especially nothing compared to 2560x1600 on the Dell 3008. Comparing a 40" 1080p TV and a 30" monitor would be like watching a film on TV with a bad signal and watching the same film in 1080p on a blu-ray disc.
2: The Pixel Pitch on TV's is a lot higher than monitors, so basically the gaps in between each pixel is wider, so as you get close to the TV you can clearly see these gaps.
3: TV's are set up different to monitors, even if you have the options to turn off all the TV processing enhancements on a good TV (like Sharpness, Dynamic Contrast, Motion & Colour Enhancements, Pixel-Plus etc...) the colours, contrast and general balance is never as good as a good monitor, it's not as accurate. And a lot of TV's dont have the option to turn off every single 'enhancement' so unlike with a monitor over DVI you dont see exactly what is being output by the computer, as it's had something done to it.
If someone recommended me using a 1080p TV for my design work i'd seriously laugh in there face ...then beat 'em up for being SO stupid.
The single thing a TV is decent for when used as a monitor is gaming, purely because of the size you can get TV's in. But personally i still use my 24" monitors over my 46" TV for this.