A couple of the collections that I was PhotoSynthing would end up hanging during the upload phase. Even after an hour of waiting - the upload hadn't seemed to have finished.
I then found that PhotoSynth writes a log of what it is doing. When I looked at the log - it said that the collection had finished uploading. But in addition it also had the following message:
"Collection completed (100% synthy), but isn't avialable for viewing yet"
Turns out the PhotoSynth system had a couple of problems in the back-end because of the amount of load that the team's servers were experiencing (link). They are fixing the problem and it should be fixed soon (link). In the mean time - you just need to check the log and if the log says that the collection has been completed - then if you visit the PhotoSynth site after a little while and check your account - you should see your collection there. (It took about 20 minutes for my collection to show up).
(Check out the spelling of "available" in the above log message!!! )