Wow, thats a new one on me.....
Are you using the newest ROM? If a hard reset won't allow it to work with either SIM, I'd try installing (or reinstalling) the newest ROM. This is a big step, and not to be taken lightly. Remember, since your phone was originaly Cingular branded, thats the ROM you need to use. If that doesn't work then...well..I have no idea. The only other thing I could think of is the SIM connection is screwed.
Anyone else?
