I found a reference to this on Ed Bott’s blog according to what he say a couple of days ago the store has opened a few hours ahead of schedule. Ed describes getting to this page from the performance settings in Vista. It is also available from the Welcome center, or via the Control Panel programs section. Or you can just go there directly. Interestingly although there is mobile software there, the Mobile device center links somewhere else in “programs and services”
This post originally appeared on my technet blog.