Though still in beta, Picasa from the Mac has most of the features of the 3.0 Windows version. However, Google emphasizes the beta label, and notes that some features like geotagging and 3rd-party printing aren't functional.
Nicely, however, they've made sure that Picasa for the Mac plays nicely with iPhoto. Photos stored stored in the iPhoto library, are treated as read-only, with Picasa duplicating files as needed, so your iPhoto library isn't ever affected.
Google says they'll be demoing Picasa at Macworld (where else?), but they also have a video tour below. You can download the beta here.