My wife just got an iphone 4 and we ported both of our old numbers over. I had the same issue. my phone was using my new ported number, but Facetime was still listing the number I'd been using on the phone for a month. Here's the fix:
Go into settings>phone and turn facetime off.
Turn Phone off, then back on.
go back into settings>phone and turn facetime back on. It'll take a second to activate and it should then be using the proper number.