What properties must an iPhone case have to avoid antenna issues? I have an AT&amp;T iPhone 4, which may have antenna problems when held in some positions (i.e. the "grip of death").  To help with this, Apple gave away bumper cases which worked around the problems when the phones were new.  Unfortunately, my old iPhone case broke, so now I'm looking at new cases. 

What properties must my new case have to be to avoid these problems with the iPhone 4's antenna?
A piece of non-conductive scotch tape is all that is required to prevent your skin from bridging the gap in the metal band and changing the impedance of the antennas.

As long as your skin doesn't short the external metal band, you are good to go, so even extremely thin gloves that are dry would work without any case whatsoever.

For more reading, see:


PC World: Apple Responds to iPhone 4 Antenna Problem 
Apple PR: Letter from Apple Regarding iPhone 4 
Apple Support: iPhone 4: Antenna or reception issues 
Antenna Engineer Blog: Apple iPhone 4 Antennas...