JCI has announced that iOS 6 is causing their lack of signal bars with their data-only SIMs. This is a common problem due to the way Docomo decided to produce the data-only SIMs. They lack a circuit switch component. There are work arounds for Android. It may be possible to do something similar with iOS, but we here focus on Android and have no idea.
Data works just fine. The problem, as described by a commenter, is that services that require a data connection shut down because the phone thinks it has no data connection. This will effect all MVNOs using Docomo's network, including b-mobile.
If you have a working iPhone with iOS 5 and a b-mobile data-only SIM, it might be a bad idea to upgrade. Voice+data SIMs work just fine.