I bought 3 iPhone 4 refurbs recently from AT&T. All three suffered various issues after two weeks of use; no audio from the earpiece being the most damning. One of them was replaced three times for this issue.
If you do have issues, DO NOT go to an AT&T store for service, repair or troubleshooting. You will end up with another refurb. Go to an Apple store. They will swap you out with a new device as a replacement...no hassle.
The warranty on refurbs is the balance of one-year from the date it was originally activated provided you purchased it from an Apple authorized source.