To be able to use (again) the Outlook features on my cell phone, I had to do the Retro Rock--that is, I uninstalled Office 2007 and reinstalled Office 2003. My Outlook calendar now works fine (again) on my cell. Of course I had to do the Retro Rock anyway to be able to convert Office files to pdf in Acrobat 8 without 14 work-around steps.
Okay, so there's a clash between Outlook 2007 and Windows Mobile 5, but it goes beyond that. If your cell has WM 5, and your computer has Office 2007, then the other Office applications that sync between your computer and cell will have their own special problems because Office 2007 has changed the format and extensions of its documents.