If you've been following developments in Iran, and particularly the Internet's essential role in reporting (and supporting) events, I don't have much to add.
If you haven't, I strongly suggest you go to Andrew Sullivan's blog and starting following him now -- as well taking a bit of time to look through his posts from the last couple of days. He's been doing a wonderful job. Far better than any mainstream news outlet, with the possible exception of the NY Times' Lede. (Although the Lede doesn't present as much detail and analysis as Sullivan does -- with the help of his readers and well-chosen links to other sources.)
One of Sullivan's readers sums up the key role Twitter is coming to play:
Ahmadinejad's and Khamenei's websites were taken down yesterday - I saw the latter go down within a couple of minutes because of a DDOS attack organised via Twitter. @StopAhmadi is a good source for tweets on this. The other important use of Twitter has been distribution of proxy addresses via Twitter. This would be how most video and pictures of today's rally have gotten out.
One amazing thing is that it seems at least conceivable that the protests would not have been as successful as they currently appear to be if Twitter didn't exist.
Sullivan suggests wearing green in support of the protesters. I think that's a great idea.