The iPhone comes out like clockwork every 12 months or so. At least the first four generations have, and I would think the fifth one will too.
At 6 months old, I start to question if it is worth buying a product half way through it's cycle, because in just another 6 months the newer better version will be out. At 8 months old, like the iPhone is close too, that means just four months from now ( February ) the next iPhone will be released in ( June ). I would rather sit it out, for just four months, and get the newest version.
I would be upset come June in four months, to see the all new shiny iPhone5, with the new dual core, better screen tech, more ram, and cool new features, while I am stuck on the now outdated iPhone4, that I just bought 4 months ago.