Brief summary - there is some stuff in the launch thread too.
There are a few slightly different issues here.
In order of concern:-
1. Old HDTV's that don't support 720p *period* will drop down to 480p. Both component and HDMI (verified by Vincey37)
2. Resistance at least notifies the user of this, where Gundam does not (worse IMO)
Bebpo
Gundam can't do 1080i but unlike Resistance it doesn't give you a nice warning.
...it just boots into 480p and you're like WTF WHY IS IT 4:3 AND ALL JAGGY?!?!
3. If your XMB is set to 1080i, but the game does not support it (like Resistance) then it will not detect 720p if it is supported and drop to 480p, forcing a manual change of the XMB setting.
4. If your HDTV can accept 720p, but not natively, you will be relying on the quality of the scaler in your TV, and some algorithms are better than others...
Obviously number 1 is the biggest issue for poor folks with a 1080i TV that does not accept 720p at all - games like Resistance will only be available in 720p.
I think the issue with Gundam not notifying that 480p is being used is far worse.
I imagine that issue 3 will be a royal pain in the ass for people who want to set XMB to 1080i and forget about it, but that have to manually reset everytime they play a game that only supports 720p.
These issues should be addressed very quickly by Sony IMHO, especially before people start making apples and oranges comparisons of 480p PS3 games to 720p/1080i 360 games - could be bad for PR.