GMail got even faster and here are some details about how they did it :
Official Gmail Blog: A need for speed: the path to a faster loading sequence
One trick that the obviosouly did, but didn't tell us, is that they send your user name asynchronously in the background as soon as you enter the password field.
This is a GMail specific optimization. It's not available on the generic Google account page (yet?) .
My guess is that they prefetch some stuff under the assumption that most logon attempts will work. Even if the password is wrong, they minimum work, that has to be done is to check whether the account is there.
The funny thing is, that I've seen this just a few days ago, when I checked how much data Google Mail sends, to get a better feeling what would a good goal for a high performance Web application.