Are you participating in the Firefox Download Day record effort? Well, I've pledged to! 17/June is THE day. What do the astrologers say? :-)
One factor that could prevent enthusiasts from switching immediately would be the extensions! Not all extensions would've been made FF3 compatible :-(. In my case switch proxy would be an example, I'll now have to look for an alternative (foxy proxy?). The guys with too many extensions that haven't been upgraded will find it difficult!