It's an insertion prank just like we used to stick All Your Base Are Belong To Us in different places. Before that was the goatse image that people used to post on forums (supposedly a significant portion of Slashdot's code is to prevent goatse.)

http://en.wikipedia.org/wiki/Rickroll has more details.

Now, it's interesting that you are seeing spam with it. I've long thought that one tactic of spammers would be to send Useful Spam in an effort to lower their junk percentage.

RE: Never gonna give you up?