iPhone 3GS sim compatible with iPhone 4?

quick question...
does the iPhone 3G sim card work in the iPhone 4?
like if I order the iPhone 4 with my mothers renew through AT&T will I be able to use my 3G sim card in it? and would I still have to activate it when I receive it or would it cancel my mothers current sim card in her 3GS?

Thanks in advance.