Hey guys,
I can put this discussion to rest as I can personally attest to the fact that the GoGo mirror series (silver) DOES, in fact, interfere with reception on the iPhone 3G and 3GS. In fact, it interferes with the reception to such an extent that the phone loses ALL reception when the case is on (from 4 bars to 0).
It is quite unfortunate because the case looks really cool.
I called GoGo and they are aware of the issue and they say it specifically affects their Silver and Platinum mirror series cases (but apparently not the gold).