Update 8 Dec 2010: Twitter blogs about this issue at http://blog.twitter.com/2010/12/to-trend-or-not-to-trend.html.
I don't usually post so directly about my work but I'm getting asked about this and it's important: Twitter is not censoring trending topics concerning Wikileaks. There's even an official statement about it from our comms team, hereby reproduced:
Twitter is not censoring #wikileaks, #cablegate or other related terms from the Trends list of trending topics.
Our Trends list is designed to help people discover the ‘most breaking’ breaking news from across the world, in real-time. The list is generated by an algorithm that identifies topics that are being talked about more right now than they were previously.
There's a number of factors that may come into play when seemingly popular terms don’t make the Trends list. Sometimes topics that are popular don’t break into the Trends list because the current velocity of conversation (volume of Tweets at a given moment) isn’t greater than in previous hours and days. Sometimes topics that are genuinely popular simply aren’t widespread enough to make the list of top Trends. And, on occasion, topics just aren't as popular as people believe.
I know the people working on this stuff.