Based on this rather self-explanatory tweet from the corporate Radio Shack account, it looks like you'll have an extra place to check for an iPhone 4 in case your local Apple, Best Buy, or AT&T stores are sold out:

YES! RadioShack will have #iPhone4 at launch. More info tomorrow.

Good for RadioShack. Seems that they usually have to wait for ages before being able to offer the latest iPhone. [TwitterThanks, Jason!]