If anyone is on Twitter, set your location to Tehran and your time zone to GMT +3.30. Security forces are hunting for bloggers using location/timezone searches. The more people at this location, the more of a logjam it creates for forces trying to shut Iranians' access to the internet down. Cut & paste & pass it on.
I don't have Twitter, but I do have a blog. Done and done. (and yes, this means my time-stamps will be horribly wrong for the near future.)
UPDATE September 2009: I have since reverted to the proper timezone. After this much time has passed, the likelihood of this sort of tactic still helping is minimal at best.