So why are you taking the framerate into account here when you previously claimed the 360 doesn't?
Unless programmers have some means to talk to the scaler and indicate the current frame rate, the 360 has to assume that it's being fed 60fps. You would get visual anomalies depending on the game if it just tossed away half the frames and assumed 30fps.
This all depends on the scaling method. Most newer displays scale 720p to 1080i correctly. I don't know how the 360 scaler does it.
Correctly for 60 fps material would be scaling each of the 60 1280x720 frames into 60 1920x540 fields. Otherwise you have to throw away 30 frames and scale each of the other 1280x720 frames into 2 1920x1080 fields. In other words you can either maintain 60fps or maintain 1280x720 resolution, but not both at the same time using only 60 interlaced fields with a lower vertical resolution then the source frames.
1080p would be the real solution to this problem. You get to scale the full 60fps with no trade offs in either resolution or frame rate.
Think of it this way. if you have 60 3 foot long boards and 60 2 foot long boxes, you can not put them in the boxes and end up with 60 3 foot long boards at the end. You'd have to trim them down to 2 foot long boards to fit them in the boxes. If you have 30 3 foot long boards and 60 2 foot long boxes, you can cut the boards in half and then put the 30 boards back together again after.