Picasa version 3.5 (build number 79.67) is available for immediate download from the Google Picasa website for users on the Windows and Mac OS platforms. (Linux users currently remain on the previous release, Picasa 3.0.) As previously mentioned, perhaps the biggest new feature in Picasa 3.5 is that the software is capable of automatically detecting and recognizing faces in images. The user can then tag the face with a name, and when the same face may have been recognized in other photos, Picasa prompts for confirmation that it's automatic detection was correct. The feature is based on the same technology that has been present in Picasa's Web Albums for some time. The faces that have already been tagged on Web Albums can be imported into Picasa, and when a tagged image is uploaded from Picasa to Web Albums the tagging information is also transferred. Once tagged, Picasa allows searching for images containing not only a specific face, but also images where two specific individuals appear together. Picasa 3.5 automatically begins scanning for faces in images when it is first executed, and if the user is signed into their Google account, then their contacts database is used to autocomplete names for newly recognized faces as they're typed in.
As well as the face recognition and autotagging capabilities, Google Picasa 3.5 also adds integrated Google Maps-based geotagging of photos. The Import Room has also been redesigned, allowing favorite images to be marked, as well as allowing for uploading of images to Picasa Web Albums and sharing with Google contacts as part of the initial import process when images are offloaded from a flash card. There's also a new Tags panel which allows quick-tagging of images with the most frequently used tags, and provides tag counts that allow the user to see how many photos have been marked with a specific tag.
Finally, at the same time as the release of Picasa v3.5 for Mac and Windows, Google has taken the opportunity to remove the "beta" label from the Mac version. This change indicates that Google now feels the Mac variant of Picasa includes "almost all" of the same features available to Windows users, although differences do remain. One example of this is that Picasa won't save alterations over an image stored in the iPhoto library, but will instead duplicate the file when saved so as to ensure iPhoto's original remains available.
For more details, read the Google Photos blog, browse the new features list in Picasa Help, or take a look at the Picasa 3.5 page on the Picasa 3 Readme blog. Google has also posted a brief What's New video on its YouTube video sharing website.