WARRANTY - Would Genious refuse service?

[Oliver350Z]

I am really confused on how their Repair/Exchange Policy work...
Specially from reading lots of information from people that were successful and people that weren't.

I am about to take my iPhone to the Apple Store, but I need some help so I dont end up looking like a fool.

Bought my iPhone Unlocked/Jailbroken from a 3rd party who had it activated with AT&T and used AT&T for service until he switched to the 3G.
(phone even came with the AT&T sim card in it).

However, I have service with T-MOBILE.

1) Will that VOID COMPLETELY my warranty even though my phone was originally activated with AT&T?

2) Is there a way around it so that they will repair my phone under warranty? (Ex.: Restore and use someone else's active AT&T SIM card)

I'd appreciate if I also could have some input (tips and hints) from people with T-MOBILE that had an experience in taking their iPhones to Apple for Warranty Repair/Exchange.

[BarcaMagic]

Restore it so they cant tell its been unlocked and jailbroken .And you can pop is someones at&t sim , but you really dont have to.Since according to warranty phone doesnt have to be activated and have service with at&t.