I downloaded and installed Picasa v3 beta a couple of weeks ago and was very excited by the features and UI improvements. My long list of issues with Picasa 2 such as lack of display of "keywords" other than through CRTL-K have been addressed. I Googled around for testimony on the Picasa WebAlbums syncing features and found some claims of problems which causes duplicate images/albums. I was little afraid to try on existing folder in Picasa which had already been uploaded to WebAlbums in version 2. I gave it a try today on a small album of some Ground Zero shots I took from PATH station last December.
My starting condition was that Picasa 3 did already have "Uploaded" (green up arrow) badging and the "View Online" link above the thumbnails worked. I would assume that if you don't have that starting condition, then you may be off on the wrong track to syncing. First I clicked on Web Sync Icon and the confirmation dialog confirmed that an existing WebAlbum was in place. I heard some disk activity after selecting OK but I did not have a visual indication of what I accomplished. I assumed that I was synchronized now and proceeded to test some edits. If I added a new keyword to a photo, the new tag appeared on the WebAlbum. (as I type this message I can't recall if I did anything to push the sync - I think it just happened after a delay). I then went to the Web Album and added a caption to an photo and saved it. Well, unless Picasa 3 opens a GMail like comet connection (he he) it really can't get an event from the web now can it. I found an online action menu (scoped to a photo or to an folder) and clicked on "Refresh Online Status". This synchronized the caption from Web Album. I then added a tag to a photo on the Web Album and again "Refresh Online Status" pulled in the web tag to the EXIF keywords.
More testing to come. I fear not finding the necessary "starting condition" described above. I don't see any reconciling features explicitly in the UI yet.