Amazon have made a very brave and clever move in light of the credit crunch. One of the first things to go at times like these is the purchase of luxuries, like books.
Lowering the minimum order value to get free delivery is a great move by Amazon.
It says to it's loyal customers that Amazon knows that times are hard but that you still want to buy things from Amazon, so it's making life easier for you. For none or infrequent users its a reason to log on and buy stuff from Amazon while it's competitors still have high delivery charges.
Amazon are clever enough to know that trying to hang on to margins is a recipe for disaster and that doing this know rather than trying to hedge its bets is a way to keep going when they'll be a lot of casualties.