Native resolution will only apply to fixed pixel displays, such as LCD, DLP and Dila/LCOS displays.
CRTs are multi-synch, such as your computer monitor. They have a maximum RGB bandwidth and can display anything at or below that number, as long as you don't exceed the horizontal and vertical scan rate maximums. (i.e you cannot display 8x6 pizels a quarter-million updates/second).
Most RPTV sets do not do 720p, but rather convert the signal to 1080i w/their scaler and run just that one resolution. They also scale 480i and 480p to 1080i and display that. Since their internal scalers are of marginal quality, this results in an unfortunate probem:
HDTV sets don't display as nice a picture as older NTSC analog sets when fed a standard 480i signal.
I can only compare ABC's 720p to the 1080i coming off the sat, but I am not at all disappointed in ABC's quality. Unfortunately, although Digital TV had the wonderful opportunity to rid the country of interlaced signals and switch everything over to progressive scan, it seems as though interlaced isn't dead and won't die.
For some things, interlacing is not that bad, but there is a reason that all computer displays run progressive - it eliminates flicker. Another thing about 720p is that is compresses better than 1080i, which may be the reason I see compression artifacts off 1080i sat feed, but not from ABC. Of course, there may be another reason, but I don't have enough sources to test this theory.
Ideally, you want a TV that can handle 720p, as you can use it as a monitor for a computer, which is great for HTPC. When they send a 540p signal at the same bandwidth as 1080i, you loose a lot of resolution you could have had with 720p.