Unfortunately all of the download day hype has been causing denial of service on the Mozilla servers. As I write this, 2 hours after the supposed launch time, the site is in the process of being updated with the new Firefox 3 logos. Unfortunately the download link still points to Firefox 2, which gives a Service Unavailable message :(
UPDATE: They have fixed the problems, and although the site is still under heavy load, I was able to get the new Fx3! Great job on the os-specific instructions :)