Sure, Borderlands 2 will be available on September 18th, and sure, you could actually go into a store and buy the game. I guess you could do that, or you can download it from PSN at launch.
The PlayStation Blogcast revealed that not only can you download it from PSN, but in Europe, you can pre-order the digital copy of Borderlands 2 from PSN. The European version will be available three days later on September 21st. However, keep in mind that here in ‘Murika, you won’t be able to pre-order the digital copy, which means you won’t get Premiere Club access. Whoops.
Source: PlayStation Blog