This essay sums up all the most popular spam filtering techniques and shows the tricks that spammers use to bypass each one. The fact that spam always looks like total nonsense is really the best way for spammers to trick filters into thinking that the email is a genuine one. After reading that essay you kind of understand what each bit of jibberish is like:
"Hmm, this email has the words 'kitchen basket' in the subject line, i wonder why"
"Ohhh yeah, thats probably because bayesian filters might give those words a low spam rating"
Spammers actually test their spam against different filters before they send them out to make sure its going to hit the maximum amount of inboxes. Spammers aren't stupid, but they're obviously not too concerned about being one of the most despised groups of people in the world right now.
BTW, the site that the essay is on is a pretty good read, Paul Graham is a good writer and knows a lot about spam and a lot of other stuff. I read a bit of it every now and then.