If you're using a [name]@mac.com email address, try using [name]@me.com instead.
Functionally, @mac.com and @me.com will go to the same place. On the iMessage settings I first put @mac.com as an email address and it gave me the 'in use' error (despite the fact that right above it is listed my AppleID as [name]@mac.com). So I used @me.com instead and it took it.
Now, I haven't verified that it works yet as I haven't messaged anyone with iOS 5 and iMessage yet, but I'll try it later tonight when I get my wife's phone upgraded (despite the fact that her new one is due to arrive tomorrow).
While they are functionally the same in regards to being Apple ID's and email addresses, when a specific address is used as a contact method for FaceTime and/or iMessage, they are not interchangeable. If you try to FaceTime with someone and you use
name@mac.com to try to connect to them, it will not work if they have only specified to use
name@me.com as the way to contact them, and vice versa.......again for FaceTime and/or iMessage.
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we had this problem yesterday.
this is what I did to solve it.
I'm accustomed to entering apple id@domain for itunes store. It asks for your apple id. so when I did it for the imessages I got the same error.
It drove me crazy then I did this.
On the 1st screen I just entered apple id and password. i dropped the @domain part.
on the 2nd screen I then entered my apple id@domain.
and it worked.
Give it a try and see if it helps.
It worked because the second time, you are actually using a different Apple ID.
See this is why apple really messed up asking people to use email addresses as Apple ID's.
An Apple ID "mickey@me.com" is just a string of characters, an account name, it doesn't *know* that's an email address. That Apple ID also has a primary contact email address, it can also be "mickey@me.com" but it does not HAVE to be, it could be "mickey@comcast.net".
So when you leave off @me, and just put in "mickey", you are using a DIFFERENT Apple ID, with a different associated primary email address. Most likely, the Apple ID of just "mickey" did have the email address you want to use for iMessage/FaceTime already associated with it. That's why it worked for Mickey, but for "mickey@me.com", it said it was already in use (which it was, by the other Apple ID of just "mickey"....