Twitter exists somewhere in the middle of the continuum between real-time, synchronous communication such as the phone and instant messaging on the one hand and asynchronous communication such as email on the other. However, Stuart Henshall and David Beckemeyer observed that a surprising number of Twitter conversations escalate to real-time. (Try doing a Twitter search on "Skype".) The result: Phweet.
Phweet allows you to invite a fellow Twitterer to a real-time voice conversation which can be any combination of Flash-based in the browser and plain old phone connections. The invitation is sent as a Tweet containing a URL which the intended recipient can click to enter the conversation. There is an option to send this URL via Twitter Direct Message, but a more interesting usage is to send it out on the public feed. Although the user interface asks for the Twitter handle of the intended recipient, anyone can answer, and if more than one person replies they are all joined in the conversation. The result - a "Tweetup".
The implementation is "Alpha" and thus still a work in progress, but it works as advertised. The actual conversation is carried by the TringMe widget and TelEvolution service which supports both Flash and SIP, although I have only tried the Flash version so far, and it worked well on my RCN cable modem. Check it out.
Thanks to Jeff Pulver for bringing this to my attention.