Currently Being ModeratedNov 19, 2012 1:11 PM (in response to Antony D\'Emanuele)
I am unsure how it's set so that messages can be received through a phone number on an iMac, and if so why it's not done on iPad.
But at this current interval in time, iMessage on iPad is only set to use the email address.
I have heard of some incidents where removing and re-adding the Apple ID can bring the mobile number up on the iPad, but to my knowledge it's fairly hit and miss, and not how the device is set.
So if you and a contact are having iMessage conversations using the Apple ID as the recpient, then you can absolutely do it on all devices, otherwise not sure it can be done there.
Best to leave feedback at apple.com/feedback