I'm a card-carrying Java-hater (haha only serious) and Lisper, I was pretty damn impressed by this the last time I watched Notch working on a LD thing. I looked it up this time and found out that Eclipse calls it "hot code replacement"

http://wiki.eclipse.org/FAQ_What_is_hot_code_replace%3F

There are some limitations (can only change the body of an existing method) so it's not quite as dynamic as Lisp + Emacs + SLIME, but it's still pretty cool. I wish Dalvik supported it somehow because from what I can tell, you can't do this with code running on an Android emulator.