Market E-Mail Address

I use a Google Apps e-mail address (i.e. it's tied to my domain) and GMail can work with this fine, but when I tried to purchase something from the Market it said it can't use this and I need a googlemail e-mail. I have one so I entered that, but now I want to use one I have set up especially for this purpose but I can't see how to change this relationship. If I stick a different SIM in, the Market now asks me to confirm the account password but it won't let me change the actual address. Does anybody know how to change this?

I'm running 1.5 on a T-Mobile Pulse.

Cheers