While this is an Exchange Server blog and not Outlook blog, I did want to call this out as it has been mentioned a few times in comments on our blog.
Office team has released an update for Outlook 2007 which addresses Outlook 2007 performance problems that customers have reported to us, especially when working with large .PST and .OST files. The relevant KB articles are:
Please note: If you are using Microsoft Outlook Business Contact Manager (BCM) - make sure to read this before installing!
Edit: Will Kennedy has posted additional information about issues addressed in the update on his blog. An interesting read!