I decided to try the open source Aperture to Picasa export plugin.
It worked for about 100 images, then died. I couldn't get it to upload additional images; I got a range of cryptic error messages.
There are 24 open defects, it was lasts uploaded Nov 2010, 3 years ago.
Don't bother with it.
Ubermind used to make a plug-in, but it's gone (I wouldn't trust the versions I can find around the net).
Once I get time to upgrade to Mountain Lion I'll presumably be able to share the library with iPhoto, I think Google still maintains an uploader for iPhoto.
Update: Google has discontinued support for the Mac uploader. I'm so glad I pay for Google storage for my photos.