I believe it's associated with your google account (coupled with some other things identifying the phone, I think).

Best bet is probably email the devs and explain the situation. They'll probably help you out...

 

The normal paid app's are linked to your google account, so if you are using a different one there isn't much you can do (that I know of).

But for Launcher Pro, you don't pay through the market, you bought that directly from him. He should have sent you an email with an uplock code, so if you still have that email just download the free version from the market then use the same code to unlock the Plus features. If you don't have the email, send a message to the dev explaining what happened and I am sure he'll send you a new code.