Probably the only way you're gonna find out is by trying it yourself. Not too many people in these forum sites have a TV that forces 1080i upconversion afterall. The next closest opinions you'll get might be from Hitachi and Tosh owners since those TVs force 540p upconversion, which may look very similar to 1080i upconversion.
Personally, I think your best bet is to wait for the Samsung HD931 and get that instead of a 480p player. That player offers 1080i upconversion via its Faroudja fli23xx chip. That's probably better than letting your TV do it since it eliminates the extra D->A->D conversion before upscaling PLUS can use that DVI/HDCP input on your TV. OTOH, you'll have to hope that your TV's video DAC is at least as good as the player's.
Also, w/ the HD931, even if you find that Faroudja deinterlaced 480p looks better w/ your TV, you can just switch to that w/ that player. It's nice to have the option between top quality 480p and good 1080i upconversion--assuming that player fulfills its promised potential. FWIW, its high $$$, hard-to-find, big brother HD1000 is getting rave reviews in these parts.
_Man_