iMessage issue - My send button is green, friend button is blue?

I have had iOS 5 for a few days now and been texting other people with iOS5 with no issues (send button blue, continuing our conversations seamlessly since before iOS 5 update)

However my brother downloaded and installed iOS5 last night. I sent him a text today which sent as a text (send button still green, thought nothing of it though) he replied, also showing as an old fashioned text on my phone.

Then I realised that this should not be happening, as both send buttons should be blue?

Then just now, his send button shows as blue, so he sent a message and it arrived on my phone as from his email address???? My send button to him is still remaining as green...

Can anyone explain whats gone wrong here? Does his phone number need verifying?

Thanks

James

Yeah thats what I was thinking..

I deactivated iMessaging on his phone, then reactivated. Going to reboot now and see.

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both now blue, and send from phone numbers rather than email addresses, perfect!