Rafe Needleman at CNet writes that his knowledgable sources say it is incredibly challenging making a realtime synchronization engine work at scale, and I do not doubt it.
Perhaps, though, a realtime synchronization engine conformant with Google Wave protocol which is limited to 50 messages per day from a total participant pool of 50 users would be would be merely an occasion of long nights and soul-searching. It would certainly be useful for those who have no sandbox account.
I found one project (through a Twitter search, thanks purserj) named pygowave-server. Are there others?
