The Mac 10.6.4 update released this evening corrects the metadata incompatibility between Aperture 3 and Photoshop. Makes the metadata more widely readable by third-party programs. Seems to work perfectly now.
10.6.4 also adds tethering for the 1Ds Mark III and the 5D Mark II. Word is that some other Canon cameras are included in the mix but Apple has not published an updated list yet.
--Here's the updated list from Apple of cameras which work with tethered shooting in Aperture
... but beware. Several people have had computer freezes and need-to-hard-restart-the-whole-Mac display crashes following the 10.6.4. update with Aperture 3.0.3. I'd hold off on updating until this has been looked at, as 10.6.3 and Aperture 3.0.3. seem to be stable.
May be an issue with NVidia display drivers as others have reported 3D game performance issues with 10.6.4. It seems to freeze the computer, or cause repeated flickering of the display, spinning beach ball of death and/or display corruption. Seems to be worse when Aperture is trying to do several things at once (like process a pile of previews AND import AND edit pics).
It's been reported to Apple and hopefully there will be a fix soon but in the meantime I've just rolled back to 10.6.3