Here's the sysomos research, which shows that there's not actually that much in the way of 'conversation' between tweeters. As for what makes for a 'good' re-tweetable tweet, here's some analysis by pistachio. It seems that tweeting in the early afternoon is the best time, oh and don't forget to say 'please'.
Update 16 March 2011: the pistachio site seems to be down, so here (for a while at least) is their article available on Google's cache.
