Even if the CU500 only works with a 3G SIM, it's no big deal. Just call your nearest AT&T COR (corporate-owned retail) store and make sure they have the capability of transferring your contacts from your current SIM to a new "fireball" (3G, HSDPA-capable) SIM. Most stores, as a courtesy, will upgrade your SIM for no charge. A new SIM might even improve your reception.
I recently broke my phone and bought a Samsung Sync off EBAY to replace it. However my non-3G SIM card was not accepted by it, so I returned it. A friend of mine has offered me his old LG CU500 (he bought an iPhone), and I was wondering if since the CU500 supports 3G can you only use a 3G SIM card with it. I read in another forum that it doesn't need it but I was wondering if anyone knew for sure. I don't want/need 3G so I don't want to bother buying a new card if I don't have to.