Actually it's not that simple, it's a BUNCH of things even if both are true 1920x1080(1200);
-the primary one now being PIXEL PITCH, which means you can still have the same resolution, and still have the supremely defined LCD pixel, but if you have 1920x1200/1080 on a 50" TV it will never look as sharp as the same resolution on a 20" monitor because the pixels are so spaced out, and so the appearance is of a less sharp image, and on many TVs it is less sharp because of the way the pixels are arranged, and the support grid.
-next is backlight uniformity, it's easier to get an even consistent backlight on a small panel than on one that 30" or over. The bigger it is the harder it is to keep it consistent, and also the harder it is to make blacks truely black.
-also bit depth support may be lower on some TVs than some monitors, but this isn't always the case, where cheap monitors might be worse than good TVs, especially if it's a TV built to support the higher bit depth of HDMI 1.3a some TVs may have 10+ bit per channel with a higher internal support, while cheap LCD sometimes only support 6 bits per channel.
-then there is the display type, which has a few issue, LCD is slow (don't be foooled by the whole 120-240hz BS [ which is subfield ] it's about true response times not the easy cherry picking of boosted contrast #s), and bigger panels are almost always slower than current model smaller size panels so more chance for ghosting or colour shifts; plasma is faster, but it has it's issues to, plasma is great for TV because similar to CRT it is built in the TV industry to have a bit of edge bleed/blend which helps reduce the problem of pixel pitch and make it a more fluid image that is well blended to cover the blockiness of a big pixel pitch (think of it like SSAA), but this isn't great for viewing text since it softens an image somewhat and lowers the definition of edges.
So really it depends on what you prefer, a really sharp/thight but smaller image or a bigger image of with some concessions?
You can get close to the same for both, but that usually means it's an expensive TV to equally a cheaper monitor.
The best thing is if they will let you plug your laptop into the ones you're considering and compare if what you're seeing is 'good enough'. If they won't let you, ask why, because they should be more than willing if you're a serious buyer that's alot of money they just said 'no' to, and I wouldn't trust a place that didn't respect that or the wishes of the customer which are that easy to accommodate.
I personally use all three (monitor, TV , Projector) for different things. But I make sure they are always good enough to do the multiple tasks
http://www.engadget.com/2007/01/19/ask-engadgethd-what-is-the-difference-between-a-hdtv-and-a-moni/