I don't think you can unlock the PPC6700 yet. If you can I haven't been able to find anything about doing that. I was actually thinking about switching to AT&T since we are on the borders of an ALLTEL network at our house, but AT&T covers us pretty strong. (1 bar with Alltel vs 4 with AT&T). I decided against it for financial reasons.
Best advice would be to sell your phone on the service you have, and buy a used PPC6700 on the service you want. Just becareful where you buy it from and read the listing very closely.