Currently, you cannot walk into the Apple Store and ask to purchase an iPhone 4 and one will magically appear. Apple leaves you with two alternatives: buy throught the online store, or wait to reserve an iPhone 4 online and pick it up when the Apple store emails you. I have a few quick tips on how to purchase an iPhone 4. The iPhone 4 popularity has made it difficult to find, sounds like de ja vu with the iPad.
The Apple Store Online is a safe alternative, but it will take about 3 weeks according to the Apple Store Online. From my conversations with the Apple Retail Stores, they are saying that the reserving in store would be a shorter wait. Note, you do not need to walk in the Apple Retail Store to reserve you can call. One you received the email from Apple notifying you that your iPhone has arrived, you have 24 hours. Reserving means picking the device up in store within 24 hours of receiving the email.
Best buy stores are also an alternative to purchase the iPhone 4. But, from speaking with customers and employees, they do not have a time frame in which you would receive your device.
All in all, the choice on how you would like to reserve your device is up to you. Personally, I would choose an Apple Retail Store as they tend to have a higher priority on receiving the devices. If possible call multiple Apple Retail Stores in the area and reserve the iPhone 4 – it would help increase you chances on receiving the iPhone 4 sooner.