iPad and service providers

I am going to buy an iPad and want to add it to my existing service plan with AT&T. I will be switching to Verizon in November though when my 2 year contract with the iPhone is up. Will there be any compatibility problems with my iPad when I change service providers from AT&T to Verizon?

Also, does anyone know if I can just add iPad service to my iPhone service with AT&T - I don't have to sign up for a new contract for a long time, do I? I hope this makes sense. Thanks for your experiences!