I have been getting this problem too. I fixed it by reseting Safari. When Safari is opened click Safari on the Menu Bar and then click reset Safari. This WILL REMOVE YOUR SAVED PASSWORDS HISTORY AND ANYTHING THAT YOU HAVE STORED!! But it fixed the problem!
Think this happens more with Safari 5.1 and certain sites.
You can choose NOT to reset saved passwords and autofill in Safari if you wish, assuming that this problem isn't linked to saved passwords and autofill, of course. I'm having this problem, too, and I've reset within the last week. Looks like I'll need to do some more testing.
Bump. I'm having the same problem in OS X 10.6.8, fully patched including Safari 5.1.2. Only happens (so far) on MobileMe. Fixing permissions and rebooting have no effect. Chrome accesses the site without issue. I know I used to be able to access MobileMe on this Mac...I'm having to assume some patch issued by Apple in recent months broke this though I use webmail so rarely that I cannot pinpoint which one.