I wrote a Lightroom plugin for migrating from Picasa to Lightroom. I searched all over the web and could not find a good solution, so I wrote one myself. I decided to polish it up and offer it for others to use for free. You run it from inside Lightroom and it imports all your Picasa Albums, people/faces, and star ratings. Your tags and captions will also be imported automatically. It's a one-time import procedure that only takes a few minutes. The instructions also explain how to save your Picasa edits (which is not automatic as implied by a posting in this thread).

The plugin is called P2Lr and you can find it along with instructions here: http://picasa-lightroom.com

I have tested the plugin on Windows and Mac, using the latest version of Picasa and Lightroom. I tested it using various test albums and my personal collection of over 33,000 photos. But other than that it has not been widely tested so it's considered beta at this point. If you use it, please let me know how it worked for you.

Note that an earlier post states that Lightroom automatically imports your Picasa folders. However Lightroom does NOT automatically import your Picasa "Albums" which are distinct from "Folders". Folders are simply the folders (or directories) on your disk. Albums are the separate collections you create using photos from your folders. Lightroom does not import the Picasa Albums. Nor does it import people and star ratings. The P2Lr plugin imports all of them.

Good luck! Let me know how it goes.