This appears to be a system that allows strangers to coerce my client into contacting their server.
Yes, people can send you false pings, which is why you need to go back through the name server and do a reverse lookup, and once you fetch the message you need to parse it and make sure it within bounds (less than 160 characters or whatever) and addressed to you (has @foo in the message text.) and is not spam (the name isn't in your killfile, is in your whitelist, akismet says it isn't spam, etc.) Rate limiting helps too, real people aren't going to send you 100 messages a second.
Personally I would go with:
User Adam sends message to Bob: 1) Adam sends message to Twitter service. "Bob>Hi." 2) Twitter check to see if Bob knows Adam. 3) Twitter sends "Adam>Hi." to Bob.
(With optimal storage so Bob and Adam can see their old conversations)
Your system let's random spammers people find out people's address which IMO is bad.
PS: Or Adam could send a message to "All>I like this soup." and Twitter then sends the message to everyone that cares about Adam including Bob.
Edit: It looks like Twitter is sending around 2million Tweets a day.
Also, all these proposed messaging architectures are somewhat flawed. Twitter isn't really "sending" messages to other people. It's more like a person's message history is bound to their account, and appropriate privileges are applied. Then, when people try to "read," privileges are obeyed and information is produced...
Am I the only one who believes the solution to Twitter does not involve a massive distributed system? Twitter is inherently centralized... people don't see that.
PS: Text is cheep EX: Slashdot is running off of ~4 computers. And bandwidth is not a scaling issue until you need a single system to handle more than ~1GB of bandwidth per second. (Saturating an OC-3 line costs a lot of money but you don't need to change your architecture to do so.)