Apple has dropped prices on its remaining iPhone 3GS units ahead of the launch of iPhone 4. The company’s online store is now offering 16GB iPhone 3GS units for $149, while 32GB models are priced at $199. Both Apple and AT&T have already sold out of their remaining iPhone 3G units; Apple will be replacing the $99 iPhone 3G with a new, same-priced 8GB iPhone 3GS model, presumably on June 24, when the iPhone 4 officially launches. Notably,…