I have tried logging it with Apple twice! First time I spoke with an Apple Tech and they tried all the usual tricks, uninstalling then reinstalling, restarting the computer, shutting down the firewall and anti-virus, none of which worked.
I then had to call back the next day and spoke to a Senior Apple Tech. They were a bit more helpful and were able to isolate the issue to my actual user account on my Win7 PC. I setup a new user called Test and logged onto my PC using Test, launched the iCloud Control Panel, entered my username and password and the control panel successfully logged in.
Then I went back into my normal account and tried the same, unfortunately it didn't work as it just sat there thinking with the loading/logging in icon spinning around.
The tech advised me that it was a bit more complicated then he thought and asked for my phone number and email address to get back to me the next day. Here is the next day and no contact from Apple in relation to this issue.
If any of you are interested the Apple case number is #253376216. You might be able to quote this and suggest that you are having a similar issue as to what I have already reported.
It is quite frustrating as I have been using MobileMe without issue for years, but as soon as I migrate to iCloud and try installing and using new software the whole thing goes belly up. Now I can't sync anything from my PC to my iCloud and hence my devices aren't syncing with my contacts, calendars, photos etc etc.
I hope they have a resolution for this soon.