Google says the new Picasa for Mac will have the same features as its current Windows version, Picasa 3. It will enable synching of photos between the desktop and online, so that edits made to desktop photos will automatically be synchronized with those online. In the Google Photo blog announcing the new ware, Google says Picasa for the Mac also "plays nicely" with iPhoto for the Mac:
Picasa takes a special read-only approach to editing photos stored in the iPhoto library, duplicating files as needed, so your iPhoto library isn't ever affected when you use Picasa.
The software requires Mac OSX 10.4 or above and as Google says, it's very much a beta and initially being released as a Google Labs product. As such it cautions that some smaller features available in Picasa 3 for Windows are not yet ready for the Mac, such as geotagging and third-party printing.
To try it out, simply download Picasa for the Mac via Google.