Facebook pretends to be twitter, and now so does Google Mail via Buzz. If these microblogs, public status notifications, or whatever you want to call it is something people really use, then it needs to be federated the same way XMPP and email is federated. You should be able to sign up for any service on any server and have your ‘tweets’ available to other service. Think how much email would suck if you could only talk to people on your service you use. XMPP still needs more acceptance, but the SMTP spec is universal now.
I’d like to see an XMPP spec for microblogs… though I guess you could argue that RSS feeds are enough for ‘tweets’.