What prevents the iMessage slider from working on a new iPhone 5? I'm struggling to enable iMessage on my new iPhone 5.

iMessage works on my iPod, but I go on settings to turn on I message and the button that slides to enable/disable it always slides back to off. (as if by force on it own and I even tried tapping it just incase it was my fault.)

I got my iPhone three days ago and it's been bugging me to get this working.
You will need to work through the various causes at will contribute to the failure of your device to register for iMessage service. 

My guess is there is something wrong since normally, you get signed in and find a password error when entering your Apple ID elsewhere on the phone. Also, a problem with SMS activation happens after you manage to get the phone to accept the slider in the ON position.

Start here http://support.apple.com/kb/TS4268 in troubleshooting and know you can also call Apple (or visit a convenient store) for more personal help. New devices, no matter where you purchased it, get free support for setting it up in the first 90 days.