This is happening fairly intermitantly and is sometimes solved by a refresh (although sometimes not).
Interestingly, I've also noticed Chrome/IE complain that "your connection to mail.google.com is encrypted with 128-bit encryption. However, this page includes other resources which are not secure. These resources can be viewed by others while in transit, and can be modified by an attack to change the behavior of the page."
(this is the experience on both my mac and windows machines).
I've disabled practically all the "google labs" plugins I have, but it doesn't seem to have helped.
So... Anyone else experiencing any of this?