Recommended PHP and MySQL Versions 
I should also take this opportunity to point out that while the ongoing vBulletin survey has revealed a very encouraging level of PHP 5 adoption, it has also highlighted a large number of customers still languishing in PHP 4 land.

I recently updated the vBulletin recommended PHP and MySQL versions to PHP 5.2.5 (or later) with an opcode cache such as APC and MySQL 5.0.51 (or later).

If you know that you are running an old version of PHP or MySQL you will find that vBulletin runs much better with the newer versions. Perhaps you could find time before the release of vBulletin 3.7 to take the plunge and get your server up to date, or pester your host to do so?

To reiterate those recommendations, we would suggest the following:

PHP 5.2.5 or later APC or similar PHP cache MySQL 5.0.51 or later