If you're using the Entourage for Web Services beta, do not install SP2. You should continue to use Office 2008 12.1.5.
We're putting the final polish on the bits now, and will have it ready to go soon. When we do, I'll post instructions for what the beta users will need to do to use the final release.
Edited on 17 August 2009: The final version of Entourage for Web Services is now available, so you should update!