Microsoft just released an update for Outlook 2007 that addresses performance issues, especially for users with large OSTs. http://support.microsoft.com/kb/933493. Several of my customers have installed the update and say their Outlook appears to be "snappier".
Microsoft also provide general guidance on how to workaround performance issues for users with large PSTs or OSTs. http://support.microsoft.com/kb/932086. Guidance ranges from "reduce the size of your mailbox" to "don't use cached mode". I love it. Aren't these the features being touted in Outlook and Exchange 2007?