Just to let you know, fetching messages from the public timeline is currently not cached. It probably doesn't make much sense either because it would then flood the cache too quickly with messages which are not likely to be requested again. This would result in messages being flushed from the cache too quickly.
On neither of the popular microblogging sites (Twitter and identi.ca) is the public timeline updated in real-time. This makes sense because the performance overhead is significant and the benefit fairly small. In light of the recent performance benchmarks I'm not sure it's wise to update the public timeline in real-time. Maybe some sampling would emulate real-time reasonably well? Anyway, sending 20 messages instead of 1 would might waste some bandwidth, but will not cause such a performance overhead as hitting the DB. Finally, the tag clouds you see on the main page are currently based on the friends' timeline, so it is not the same for different users. It could be generated out of the last messages of the public timeline, but then it's all again about fetching the messages. Not that I'm against having some real-time functionality on the front page, but I just wanted to note that this goes beyond what just the new UI involves.
