Should be able to if your phone is unlocked. Also straight talk has unlimited everything and I have read a lot of people are using it with their iPhone and the iPhone doesn't have to be unlocked.
If you go to Straight Talk you don't need to have you iPhone unlocked. Just make sure you get an AT&T Compatible SIM (micro SIM for 4 or 4S, normal for 3GS).
Straight Talk and Net10 are owned and operated by the same company. What I want to know is if I could stick my micro sim in my iPad. That would be useful, as Net10 does not allow your phone to tether.