The long-awaited vBulletin 4.0 was recently unveiled at vbulletin.com. The developers are calling this a Beta but in truth it's pre-Beta because things are still changing. A true Beta will be released mid November and I'll be taking a close look at it.
What's got everyone in a tiz is the developer's decision to "progressively enrich/enhance" vB. Effectively, that means using some neat CSS 3.0 tricks that are not understood by IE. So users of Firefox see nice rounded corners but IE users see ordinary rectangular boxes. See the vB must work perfectly also on IE thread. At the time of writing, the thread has 132 posts and it's still growing.
Take a look at the vB forum in IE and FF to spot the difference.