If you've been watching other blogs, or the recently published KB articles, you probably have seen this already, but I thought I'd point it out for the rest of you. Last week we released an update for Outlook that fixes some issues that we were not able to fix for Outlook 2007 RTM. Included in this fix are a couple of performance tweaks which help out some of the performance problems users were reporting when running with a large PST or OST file and Outlook 2007.
More details of the update are avaialble in the associated KB article: http://support.microsoft.com/kb/933493.
This won't fix everyone's performance problem, but it does make several common causes for performance issues in Outlook 2007 less of an issue.
Warning: If you're using Outlook 2007 with Business Contact Manager, you should read this article first. There is a follow up patch you need to install to maintain the full functionality of Business Contact Manager.