Here is why
I think Apple lowered the price.
http://www.engadget.com/2007/09/13/iphone-now-software-unlocked-in-32-countries-and-69-carriers/
It's been known that people all over the world have been buying them at the original place waiting for the SIM unlock to arrive.
A free SW unlock was announced last week and a GUI for it just came out.
So far it has been tested and confirmed to work in 42 countries and 90 carriers, (title of page is incorrect, look for update in article).
I think the global demand was so huge and people have been buying them with no promise that it will ever work and Apple knowing, (maybe hoping) that it would be hacked wanted to take advantage of the expanded market.
Remember the price adjustment came on the heals of 3 unlocking announcements.