For now, Picasa only works in Windows and Linux, using WINE. There's also a Mac uploader for Picasa Web Albums that lets you upload photos from iPhoto.
Google is increasingly concerned with supporting other operating systems than Windows: a single Google software works only in Windows - Google Talk, even though there's a limited web version.
|Google Toolbar||yes||yes - Apr. 2006||yes - Apr. 2006|
|Google Desktop||yes||yes - Apr. 2007||yes - June 2007|
|Google Earth||yes||yes - Jan. 2006||yes - June 2006|
|SketchUp||yes||yes - June 2006||no|
|Picasa||yes||no||yes - May 2006|