First question is:
do you have an AT&T iPhone?
If so, I suggest you put your iphone in recovery mode, then restore through iTunes. If that does not put you in the latest firmware (1.1.3) plug it into iTunes again and Update it.
With a defective 1.1.3 iPhone, unlocked, if you can show them your problems with wi-fi, they'll very likely replace it.
If you don't have an AT&T plan, though, I don't know if you can safely take your phone to use apple warranty.
Since you're from outside the U.S., I reckon you don't have an AT&T iPhone, so, I guess my reply is useless.
