This is awesome in so many ways. It exploits a well researched problem of computer image analysis, which previously was defeatable because of too small a set of source images. Microsoft Research solved this problem by hooking into Adopt-A-Pet's database of cat and dog images, which is constantly changing and has 2 million unique pictures at any time.
And, they put links into their stuff to actually go and adopt the pets!
Asirra: harnessing the power of teh cuteness to defeat the bots!