We had a customer report to us that they couldn't check out because the paypal button in the cart was broken-- it just didn't do anything. We tried to reproduce it with their conditions, and it sure felt broken to us-- the button just *does nothing.*
The root cause, "of course" is that it will always do this if you haven't selected a shipping method. (Or, if you haven't entered your zip code for tax.)
I presume that this is the normal and expected behavior for the cart. However, it is not very graceful. Is there any way to show the user a message, indicating that they need to fill out the form before the button will work?