While this comment and its siblings do mention methods that can, in theory, be used to launder bitcoin, it's still really difficult to do it correctly. People vastly overestimate the privacy benefits of bitcoin. In many practical cases, bitcoin is probably one of the least private ways to get payment; every transaction is publicly recorded and can easily be traced through the blockchain without subpoena, warrant, social engineering, or any justification whatsoever. This is a treasure trove for law enforcement and will likely allow them to find criminals that never would've otherwise appeared on their radar.
Some tumblers are honeypots and many are scams that will skim the top off your btc while barely doing anything to your coins' "taint" rating.
Even if one's understanding of bitcoin and practice of opsec is impeccable, there is still the need to worry about trading partners out to the nth degree; one weak link somewhere in the chain may very well lead back to an otherwise-meticulous user. One also must worry about getting discovered through some other means, like getting a computer booked into evidence and having the bitcoin wallet found and a legal order entered compelling its decryption.
If law enforcement finds out someone unsuccessfully engaged in an attempt to conceal their possession of bitcoin, that person is adding years to a potential prison sentence with conspiracy charges, money laundering, tax evasion, etc.
Bitcoin is awesome for everyday use. Anyone who wants to use it for "underground" endeavors needs to do some serious homework, and not just plunge in believing "it's anonymous currency" or "it's anonymous as long as I send my coin to a tumbler after getting it".