I enabled the debug menu in safari using a hint I found here about a week or so ago.
I frequently visit some site that are aware that they do not fully function with Safari for some reason or another. Most of these sites involve shopping carts or other assets management functions. One example would be Photospin.com -- if you have an account, you would not be able to download anything using Safari. Your account would think you were not logged in any more once you left the original login page. If you would try to log back in, it would tell you your account was in use. This never happens when using Opera or Internet Explorer.
Well today I got to fiddling around in the debug menu and changed my user agent from "Automatically Chosen" to "Mac MSIE 5.22" -- and it worked. My misbehaving websites all function properly now. What I have noticed is that this seems to work on a site by site basis. I am using four tabs right now, and am able to set a different user agent to each one.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030904141111821