I have been waiting for this moment for years! (Download here)
Unfortunately, moving files between the PC and your Mac won't be so easy. Picasa stores the picture settings in a file called picasa.ini on the PC, and the file is called .picasa.ini on the Mac. Also, the originals get stored in a folder called Originals on the PC and .picasaoriginals on the Mac.
I did a test to see if the PC version of Picasa will acknowledge the Mac ini file and originals folder, but it doesn't, unfortunately.
This incompatibility is a HUGE disappointment. A simple fix would be for both versions to recognize both formats and adapt appropriately.
Google, are you listening?