It wouldn't be hard to prototype a coin mixer using the raw transactions API and a centralized web service (accessible via Tor, if you're worried about it recording your IP address).
If you want to do a really good job, though, you'll need:
+ Lots of people participating and/or
+ Lots of time so your mixes are spread out over time
The 'lots of people' will take time, because the mixing code needs very high trust since it'll be spending your coins.
The 'lots of time' might be a practical problem, because the mixer needs your wallet to be unlocked so it can sign mixing transactions.
Somebody should create a working prototype and then for a Bitcoin Foundation grant to fund the web service for a year...