I've been beta-testing this update for a little while and it seems to make quite a difference to the performance of Outlook 2007 (especially at startup) when you have large PST files, or a large offline cache of a mailbox. The Outlook team released it live, yesterday.

http://support.microsoft.com/kb/933493

I'd highly recommend giving it a go - it's one of those updates where you might not really notice much positive improvement after you've installed it, but you do