Due to advances in blockchain research, it was shown that significantly lower block times were possible and perhaps beneficial given the current connectivity of the internet. One of the potential risks of a low block time is a higher rate of orphaned blocks (competing blocks that do not make it into the main chain). To counter this, a GHOST protocol is used which pays for these orphaned blocks (known as uncles), adding to the security of the main chain. Instead of the main chain being "longest", it is instead "heaviest".
During the development of Ethereum, several different times and targeting algorithms were tested, eventually settling around 15s. Due to the specific total difficulty calculation used, the actual time between blocks is currently around 17 seconds, however a change scheduled for Homestead will hopefully alter this closer to 15 seconds. This is only proposed as a temporary solution, as Ethereum intends to transition to proof-of-stake, potentially offering near-instant finality.