Why aren't iMessage messages being delivered? They say 'delivered' but are not received by the recipient.

Twice in the last three days I have sent my girlfriend with an iPhone 4S (mine is an iPhone 4) on IOS5 iMessages. On my phone, the 'delivered' message shows up, but she never got the messages. And vice versa. Is there a problem with messages not being delivered on the iMessage platform? SMS is rock solid, and my, and every other messaging customer's, expectation is that if Apple tries to step into that role is that it will be equally reliable.