i had the ifrogz blue and black case, bought it from at&t for about 30 bucks even before i had my phone, but now i let my gf have the case for her iphone and i just use screen protectors, works fine for me
They all "fix" the signal issue as long as it covers the antenna gap on the sides. Anyways, I have the Incipio Gel Acrylic (ATT) and got the free one from apple the Speck Pixelskin HD. Both $30.
Just make sure you don't get a Aluminum case
my alum. case ALWAYS gave me 1 to 0 bars, and I thought it was my iPhones problem until I compared my phone side by side with a friend and his was getting 4 bars while I was getting 1