Can I use an old straight talk SIM card with an iPhone 3G?

I have an iPhone 3G don't like AT&T and have an old SIM card from a straight talk phone can I use the SIM card with the iPhone and just buy the airtime card

Nope

Na…

If the AT&T sim had came along with the iPhone 3G then it is not possible for you to use Straight Talk. As your iPhone is locked to AT&T, you will not be able to use any other GSM sim until you unlock it.

At present according to the DMCA you will not be able to unlock your phone. There's a petition "We the people" filed to make unlocking legal. And it is opened for people to sign until feb 23.