IPB has had about one security patch to it's core files since 2.0.3 came out. Once vBulletin 3.0.3 came out, there was security hole. That made 3.0.4. Then there was another. And then another. This makes vBulletin 3.0.6, while IPB is at 2.0.3 and still better.
I bought IPB for my website, and I really prefer it over vBulletin. It works, it's faster than anything, and it handles a lot of things.
(Yes, I know this topic is kinda old, but I had to make my point.)