IPhone :: IMessage Suddenly Won't Sign In On 4 And IPad 2

I cant find an iMessage or even just plain old iOS help section. Here's the thing. I have an iPhone 4 (16GB) and an iPad 2 (WiFi only 16GB) both running iOS 5.1 Until last week all was fantastic, then I noticed that during an app purchase via App Store, Apple asked me for extra "security data" and the app purchase went fine. Since then however, my iMessages have not been received by ANY of my recipients. A couple of other iMessage users found they couldn't reach me by iMessage either.

I went into settings - messages - and found my iMessage was logged out. I re-enabled it and got a message that "my Apple ID had changed" and I was logged out again! I have checked and triple checked my Apple ID, have even reset it and checked the new one too.

I can make app purchases from the App Store, and can use home sharing for my music and videos so I KNOW my Apple ID is right. But for some reason every time I try to turn on iMessage it refuses my Apple ID and Password. My iPad2 is doing exactly the same and tells me to check my email as they've sent me a "verification link" but it never comes through. I have checked the email address and it's fine. All of my own emails come straight through. In fact I've just joined in here and the welcome email just came through on the same email account

Info:iPhone 4, iOS 5.1, Also iPad 2 on iOS 5.1