The Short Version: Depending on the network environment, network authentication may block iMessage text messages. In addition to blocking messages to and from iPad and iPod touch devices, it will also result in blocking iPhone text messages. The simplest work-around is to disable iMessage in Settings -> Messages or just disable WiFi.
I migrated our MobileMe accounts to iCloud the day it was released, after updating my iPhone to iOS 5. I was using two accounts. One account is my primary and I synched mail, calendars, contacts, etc using it. I had another account that I only used for e-mail.
Overall, my transition to iOS 5 and migration of the accounts went without any major issues.
Well, that was the case until I tried to use Pages, Numbers, and Keynote on the iPhone with the new iCloud sync capability. For each app I turned on the “Use iCloud” option but every time I opened one of the apps the setting reverted back to off and in the iCloud Website it just showed the intro pages for the apps.