People who do this are morons, and more often than not angsty teenagers in denial. Playing around with other peoples infrastructure does not make you cool, if you do it to big enough guys it does make you likeley to get sued for damages though, see nessun et cetera (And yes, phishing passwords and defacing web sites is a form of denial of service, and if you're in America the I'm sure if you twist the PATRIOT act around a bit, you can also work criminal charges in there). Now go find a more productive hobby.

(Side rant, when did the knowledge about how to deface a page with some style and class get lost? Is swastikas and goatse all the creativity you people can muster?)