The story leading to this announcement is also a great example of how public opinion of a product can quickly swing the opposite way, when a company attempts to not only create a closed system, but then also tries to punish customers, who used 3rd party applications, by turning their expensive phones into "bricks" (which is what happened during the recent 1.1.1 software update).
Pressure on Apple from customers and bloggers alike very quickly led to this turn-around in strategy - and this can only be a good thing or both Apple and its iPhone customers. Having the iPhone be a true mobile platform - similar to Palm, Nokia, and Windows Smartphones - is key to any long-term success of the iPhone product line.
While I haven't personally installed any 3rd party apps on my own iPhone in the past, there are certainly several that I am missing since I switched from a Windows Smartphone to the iPhone this summer - and I am looking forward to the ability to get this extra functionality back next spring!