Technically, Cingular doesn't have to honor any sort of warranty in the store. Cell phone carriers offer in store warranty replacements as a convenience so that cell phone users don't have to ship their phones back to the manufacturer and be without a phone for 2-3 weeks. However, as long as the phone is purchased new, the manufacturer's warranty would be good for one year from the date of purchase. As for Cingular helping you if you bought your phone elsewhere...I don't see why they wouldn't. I guess it would depend on the store more than anything else. Some stores (this is all carriers, not just Cingular) suck at customer service and refuse to help people that didn't buy their phone from that store. If you had a problem and were not helped at one store, try taking it to a different store. Or calling customer service. You'll get help from someone for sure.
That was a bit long winded. Sorry, I really should be in bed right now...