Below is a list of the weeks best-selling books from Nielsen. Note that the best-seller is about iPhone developement. It is rare for books as technical as this to outsell books with much broader appeal and hence this is a lead indicator that developer interest in the iPhone platform is mushrooming. Most of the other titles on the list are about mass market apps and operating systems.
This comes in the same week as rumours that the iPhone is going to be increasingly focused on gaming and talk of a $19.99 premium section in the iPhone Appstore for games. Maybe that is why developers are getting excited.