Found a website which has a list of 1080p PS3 and 360 games.
According to them:
At the time when the article was posted:
PS3 has 26 "Native 1080p games available for the Playstation 3"
Xbox360: "Native 1080p games available for the Xbox 360 include the following
6 titles.... Box advertised 1080p games available for the Xbox 360 include the following
35 total titles. ... The titles below show 1080p on the back of the game boxes but have not been confirmed by third party sources as being native 1080p."
However there is a confusing part on Xbox360 side:
"- All titles have been checked as being 1080p compatible firsthand by the back of the game box and/or by owning/playing the game at 1080p resolution.
Unlike what is happening on the Xbox 360 game boxes, Sony doesn’t advertise 1080p on the back of game boxes unless it is a truly native 1080p title. ... The Xbox 360 list of games advertised on game boxes is longer, but there are a small few number of native 1080p titles on the Xbox 360."
Information surce: http

/www.makeyougohmm.com/xbox360-1080p-games/
Though the reliability of the information source (www.makeyougohmm.com) is not unknown, if their information is correct appearently there are quite a few so call 1080p Xob360 games may not actually being native 1080p games.