I too faced the same problem . I was using imessage in ios6 and when i upgraded , There were issues like ,
check network connections and Try again . , Connect to itunes for Push notifications . It was irritating and annoying for everyone i guess
Steps to Resolve the i meeage issue
1) Connect your phone to P.C and Back up your datas to P.C via I tunes
2) Once the back up is complete . Connect your phonbe to Wifi or 3G cellular network and Boldly go to Settings-->General --> Reset--> Erase all datas
( Do not worry if you had backed up your datas )
3) Iphone will reboot . it will take some time
4) Then your iphone will start . Click next and you will be prompted for i message activation . Give your apple id and Sign in
5)Keep your phone connected . In P.C you will be prompted if you want to restore ( EOther wise go to I tunes --> File --> Device--> Restore back up
It really worked for me .🙂
Hope this is clear.
Thanks