Just in time for the holidays, Radio Shack is cutting prices on the iPhone 4 by 50 bucks. That means the 16GB iPhone 4 is $150 and the 32GB iPhone 4 is $250 (for new or upgrade eligible customers). That's probably going to be the best deal you'll find this year. The offer is only available in brick and mortar Radio Shacks, so call your local one to see if they're offering it.
iPhone 4 (16G):
Regular Price @ RadioShack = $199.99
After $50 savings @ RadioShack = $149.99
Final price after trade-in credit of 3G ($75) = $74.99
Final price after trade-in credit of 3GS ($125) = $24.99
All depends on condition of course, but they're offering some nice trade-in rates compared to the other's. Target offered me $50 for my 3G and it's in top notch shape.