I saw that iCloud.com has gone live today and I was able to log in using my existing Apple ID. Aside from being able to look around, I'm not sure that there is much more we can do until we get iOS 5 installed on our iDevices or a update to Mac OS X that replaces the MobileMe control panel with the iCloud control panel (I'm assuming that there will be a new control panel or other software integration with the Mac OS X or Windows PC).
If you are registered as a paying Apple software developer, you will be able to download some additional API tools for building into your applications.
I was able to login to iCloud.com from my MacBook only. When I hit iCloud.com from my iPad, I was greeted by a beta page.