To help quell the uproar over the reported iPhone 4 "deathgrip" issue, Apple is offering a free "bumper" case for every iPhone 4 buyer, (normally $29). Though Apple says they will provide cases from other manufacturers to deal with a shortage of the bumpers. iPhone 4 owners who already purchased a bumper, will receive a refund.
The bumper will solve the "deathgrip" problem by preventing your skin from interfering with the antenna located on the left hand side of the phone. It will not help with general issues related to low signal strength in your area, which should be easier to see after Apple's recent fix for signal strength measurement.