Well, I got my invitation.  I have one album online right now: http://picasaweb.google.com/ariankulp.  It's nice so far, but hardly a Flickr-killer at this point.  I don't see tagging yet (though labels or keywords from Picasa might filter through).  I tried creating an album in Picasa, added some photos, then uploaded them.  From the web site I added captions, then downloaded them back to Picasa.  I thought/hoped that the new captions would import, but leaving the rest of the album alone.  It looks like it had no effect on the local version.  I'll keep messing with it.  It's a simple, elegant interface (as Google is so good at), but a bit minimalistic at this point.