Is this an achivement?

Apparently not an achivement for killing him, but it sure does piss Kel'Thuzad off.

Maybe the achievement is called 'Mr. Bigglesworth kills' because you have to get him to kill... that would be so funny.

You can use Critter Bites on him and have a little pet Bigglesworth follow you around for a while. =]

Apparently, according to myth, if you kill him your whole raid is "cursed" and you will all wipe. Unconfirmed.

This is bull. I tested it myself jsut now and all I got was a nice repair bill, boredom, and dissapointment.

It's not an achievement, it's a statistic, like how much gold you've collected or how many hugs you've given.  Thus it's in the 2nd tab of your Y interface, under "Dungeons & Raids"- subcategory " Wrath of the Lich King".  It's just past halfway down, right after Kel'Thuzad 10-player kills.