I want something that's future proof (at least for a few years) and I want all my sources to match my display devices. If the ATV was 1080i capable we wouldn't be having this discussion. But since it's not why not have the next version be 1080P capable instead of just 1080i? Even $80 upconverting DVD players output 1080P. I doubt there is much of a cost difference to upgrade the ATV to 1080P vs. 1080i.
BTW, good discussion.
I'd take a progressive signal (720p) over an interlaced one (1080i) any day, but that's just my opinion. There is a substantial difference in processing power needed to decode a 1080i versus a 1080p signal.