Thanks to Mel who sits right next to Lydia, the Program Manager for the Windows Mobile Device Center (WMDC), we have the answer for your woes. Go over to Mel's post to read up his extensive explanation about why you can't run WMDC on Vista RC1, for now.

This paragraph describes it all:

There's just one little problem. If you try Windows Update today, it will not detect the WMDC download. This is because one additional step needs to be performed to allow this detection to take place. Unfortunately that step is restricted to internal Microsoft beta testers at this point. The friendly Program Manager for WMDC, Lydia O, sits in the office next to mine. So I asked her what was preventing external RC1 testers from using WMDC. She explained that they're still working on some critical issues which make it necessary to restrict public testing. Judging from the steady stream of people walking in and out of her office all day, it looks like Lydia's team is working as hard as possible to resolve those issues and release WMDC publicly to Vista RC1 testers very soon