When I buy songs at the iTunes Store, I use the shopping cart as my "wish list" storage location. I see something I may want to buy, and I add it to my cart. I blindly assumed the Store preferences setting in iTunes to use the shopping cart would also apply to the App Store. That is not the case.
When you click Buy in the App Store, you buy immediately. (You can see what you've purchased in the Applications section of iTunes 7.7's Library. If you can't see the Applications section, you may have disabled the Games entry in iTunes 7.6's preferences; Applications replaces Games.) Before I figured this out, I'd accidentally purchased two things. Thankfully, one is a game I'm interested in, so that's not so bad. The other, though...anyone wanna buy a never-read e-book copy of A Tale of Two Cities? It cost me $0.99, but I'll let it go for only $0.39! :)
So until/unless Apple modifies iTunes' preferences to allow a shopping cart in the App Store, be very careful with that Buy button. You will purchase whatever you click immediately, without so much as even a confirmation dialog. (And because most apps are tiny, they'll be done downloading before you can even think to cancel them.) I'm just very glad I didn't click buy on the $69.99 ForeFlight app -- although as a pilot, it looks very cool!

