There is a new way to check the availability of the iPhone 3G at your local Apple store from Top Muffin. Their iPhone Availability Tracker is updated every 15 minutes with data from the Apple website. That being said, they do advise you to call the store before heading out.
Today it shows that the iPhone 3G is available in approximately 45% of all Apple stores countrywide:
8 GB Black 3G iPhones are available at 84 of 188 stores (45%)
16 GB White 3G iPhones are available at 101 of 188 stores (54%)
16 GB Black 3G iPhones are available at 75 of 188 stores (40%)
In my local stores, stock levels are almost nonexistent. The 8GB is out of stock and the 16GB is only available in 2 out of 5 stores.