Data plan for tablets

[RTS2]

I know some of the tablets coming out are going to work with a cell phone data plan like the Ativ smartPC that AT&T is already selling and I think the Lenovo Thinkpad Tablet 2 will... Anyway, I have an iPad 2 that is still on one of the original unlimited Data plans and I was wondering if I'd be able to use this sim and maintain the unlimited data plan on it or is it locked to the iPad? If I can keep it, how would I go about getting a device that supports sims as I don't think the ones in store do.

Thanks