Over at UNEASYsilence that have quick tutorial on hacking your iPhone to support two different numbers by using two SIM cards. Basically this involves using a dual SIM card holder and manually trimming down the cards to fit into the adapter. With one kind of adapter the active SIM card automatically switches each time you turn the phone off and back on. If you have two AT&T SIM cards you don't have to do anything else to get two number support.On the other hand, if you want to use non-AT&T SIM cards you'll want a different adapter and then you'll need to jailbreak, unlock and activate your phone with ZiPhone.
As Dan notes this is not for the faint of heart since it involves actually cutting the SIM cards and the hacking the iPhone with ZiPhone. Nonetheless, if you want to use your iPhone with two different numbers and you aren't afraid to get your hands dirty, head over to UNEASYsilence for the step by step.
Update: I'm neither endorsing nor not endorsing the use of ZiPhone. I'm reporting the methodology Dan used. Also the support for two SIM cards is apparently built in to the adapter, not directly in the iPhone OS which rather has support for SIM applications.
