Why you should test your pages in more than one browser
I've always been an advocate of testing web pages in multiple browsers to make sure everything works.
Now there's new data to support that approach:
As of January, 2009, Safari (regardless of operating system) accounts for 8% of all page views (so one in twelve page requests comes from someone running Safari), and Firefox over 21% (more than one in five).
So if you test only in IE (68%), then it's possible that one in three of your visitors may encounter problems with your site.
Of course, that works the other way as well--don't test only in Firefox or only in Safari. (That tends to be the mistake I make.)
Also, of course, your visitor ratios may vary depending on what your site is about. I imagine that MSDN, for example, has significantly more than 68% IE users.