Yes I know about Ambarella, their chipsets are actually better for video, but it's not possible to get anything from them.
Some faster TI chips seems to give decent quality on smartphones although no 1080 yet.
The BeagleBoard XM has a pretty "beefy" SOC even if the card is a bit big, it's rumored to achieve nearly 1080p25. It's on preorder so far.
The LeopardBoards are just slightly faster than the Gumstix but apparently allow 720p30. I'm unaware of the bitrate however...
What would be interesting is getting our hands on a Ambarella SDK unofficially or have someone who has it
I'm pretty sure we can boot those Camileos and friends and modify them (well at least the ones with the A5, I'm not sure if the A2 runs linux). (no DMCA in my country)
Actually, in theory we should be able to force them to release the source code parts that is GPL which already gives a lot of hints, but I'm sure they'll refuse without a trial.