I am having the same problem. Got a new iPhone5 and activated it under one line. This has been verified by the Cust Service on the phone too. However, the online Activate/Switch Device option says there is a pending transaction and I need to Activate the phone first, before attempting to Switch.
Has anyone found a way to get this type of issue resolved. I got this phone on Thursday and it is Monday now, enough time for the Device to show Activated status. The new phone is also working on the line where it was Activated.