Will a AT&T sim/service work in/on my t-mobile iPhone 5?

AT&T has recently opened their prepaid plans up to iPhones now and I have read a lot that AT&T iPhone 5 will work on t-mobiles service, however I want the exact opposite. I need to know if my t-mobile iPhone 5 can/will work on AT&T service?

I don't think AT&T would because they would want more many and they would say that you can't and make you buy a whole new package