I missed this thread earlier. It's a good idea to identify which HDTVs are easily supported.
As 57 pointed out, most of the warnings in the user manuals relate to either support or warranty concerns of the manufacturer.
However, it is possible to hook up an HTPC to ANY HDTV that has component inputs. Repeat, ANY HDTV with component inputs, irrespective of what the manual says. This will allow you to send 1080i or 720P to the TV, depending on what it supports.
The caveat here is that if you intend to use it as a computer desktop you need to be ready to mess around with all of the following: Powerstrip, your TV's service menu, and your PCs graphics card driver/configuration panel. This only applies to component inputs since there is no feedback from the TV to the graphics card via component so the graphics card will do what you tell it to and in the analog world of component, you can make this work with any HDTV. The question will be how much overscan you are left with.
Support for true PC connectivity, i.e. DVI, HDMI, or VGA is often trickier because the TV sets will report back supported resolutions to the graphics cards via the DDC interface and this will limit the graphics cards output capabilities. All is not lost however, as I believe most of the newer driver support the ability to ignore the DDC information, essentially allowing you to force whatever resolution you like. That being said, not all HDTVs will sync to a forced resolution.