A massively scalable, highly redundant version of Twitter can be built using the P2P feature of Flash 10.
For pennies, too.
The devil is always in the details but this is something that can be conquered now, thanks to Adobe.
Another way of saying it, building a massively scalable, highly redundant Twitter clone is not exactly trivial but it can NOW be done 800 times easier than having to write your own P2P layer not to mention having to ask people to download a PC client.
Adobe has just changed the game by making P2P technology accessible to all Flex/Flash developers.