Update: I heard from someone in Tehran; Iran's Internet connectivity is apparently considerably better than I had thought. I still, mind you, feel that DDOS is a very poor form of protest, but it is at least less likely than I thought to cause serious disruptions for normal people.
People on Twitter are encouraging participation in a distributed denial of service (DDOS) attack on an Iranian government website, to protest the outcome of the recent almost-certainly-rigged election. This sounds like a very nice form of peaceful protest; it is anything but.
Politically motivated DDOS has been going on since public Internet access was available. It's old news. Even the tech media can barely be bothered to report on it these days. And it does no good. Does Khamenei really care if a government website is a little slow? Will it persuade him to have proper free elections? Of course not. It won't even be particularly embarrassing; there's not a major government who it hasn't happened to at one time or another.
So, they don't do any good. In this case, interestingly, it could actually do some harm.
You see, it seems that the offending website is actually hosted in Iran. Iran is not your modern open developed nation with gigabit links coming out of its ears. It does not have unlimited transit, and it is likely that all its transit it through one or two carriers. If these links are overwhelmed by armchair protesters DDOSing the website, then Internet access from Iran to the outside world may be disrupted, and it's even possible that the carrier, which will also be catering to other under-developed nations in the region, will simply pull the plug to protect the rest of their network.
This is a particularly large problem because the Internet is probably about the best source of uncensored news in the country at the moment. Iran, of course, does censor the Internet, but such censorship is never entirely effective. Its current attempts to jam international radio and regional satellite broadcasts, on the other hand, seem to be going rather well.
So, you can click the handy ready-made DDOS link, and feel smug about making Khamenei cry. You may, however, be preventing actual Iranians from getting real news about the current situation. Your call.