Currently Being ModeratedAug 26, 2012 8:13 AM (in response to Tristan49)
On your iPad, go to Settings app, select Messages. Check that iMessage is turned on.
The 'Receive At' address should be your Apple ID email address. If not, change it to be that.
On your iPhone, also go to Settings, Messages.
Scroll down to 'Receive At'. The default will be your phone number. You need to add your email address if you haven't already done so. For messages to sync, this must be the same email address as your iPad, i.e. your Apple ID email. In 'Caller ID', set this as the email address, NOT your phone number.
On your iPad, compose a new iMessage to that recipient. From then on it should sync all your messages and theirs on both devices (but the old conversation on your iPhone won't appear on your iPad.)