here are some tests, using silent pics and converting them to see what they look like. this is using the information roaldfire put on the 500d vram page.
While Recording
0x44000080 / 0x4600008
0x44000080:
resolution: 1576x630
pitch: 3152

wrong address, looks like this in all 3 recording modes (used the same resolution size, but doesn't matter because it's obviously not a valid buffer address).
0x46000080:
resolution: 1576x630
pitch: 3152
1080p:

720p:

resolution: 720x480
pitch: 1440
480p:
(can't convert file? 422-jpg.py thinks it's a hi-res silent pic (2x1) but fails saying the other parts are missing. only happens in 480p mode.
resolution: 1576x1048
pitch: 3152
1080p:

(looks good!!!)
0x4FCB6C00
resolution: 720x624
pitch: 1440
1080p:

720p:

480p:

notice there are very very slight differences between them. It almost looks like zebras? I didn't have them turned on (nor should they be showing up in the buffer?). Odd..
I also note that magic zoom doesn't display a clear picture when i turn the camera on and start recording. I must first close the magic zoom window, stop recording, start recording, and then when i open the magic zoom window (preview for how things are gonna look in the silent pic basically), everything is clean then. This only happens for the first recording after turning the camera on. happens in every mode too (720, 1080, 480).
0x41B07800 / 0x43738800 / 0x43B48800
0x41B07800:
resolution: 720x424
pitch: 1440
1080p:

720p:

480p:

0x43738800:
resolution: 720x424
pitch: 1440
1080p:

720p:

480p:

0x43B48800
resolution: 720x424
pitch: 1440
1080p:

720p:

480p:

0x4d02800
resolution: 1280x732
pitch: 2560
err70 once, and video buffered out on one. silent pics won't convert like the earlier 480p buffer (none of them work this time though). magic zoom looked like a wrong address pic, so i dont' think this address is valid (at least all of the time).
0x48000080
resolution: 1576x1048
pitch: 3152
1080p:

720p:

480p:

(wrong resolution for buffer)
so that's that, every known/documented buffer on the 500d vram page has been tested in recording mode, so we know what works and doesn't now.
tomorrow i'll try to do the same in movie mode idle, and hopefully have the hd stuff all squared away, so that it works in recording as well

no guarantees for anything hdmi related yet, i haven't even tried touching that (i need to get a cable first).