CHDK works, kinda-mostly:
All I’ve really tried is raw imagery, which creates DNG files of some hugeness.
Ixus90 / SD790 Porting/details thread has more (you need to log in to see the firmware files). It’s not ready for general usage yet, but shows promise.
One reply on “CHDK on Canon PowerShot SD790”
Looks like it just hit beta, which means that the code is stable and part of the nightly build process. Yay!