You can. I'm not sure it achieves a great deal. TOR is about trying to hide the fact that two points A and B are communicating. It makes it difficult to say that A definitely talked to B, or that B was talked to by A.
Bitcoin is a P2P protocol, and all you are transferring is block chain data. I'm not sure having an open connection to the TOR network looks any less 'suspicious' than participating in the Bitcoin P2P. It just means that instead of saying you are definitely looking at Bitcoin, you now have the possibility that you are looking at Bitcoin, or Silk Road, or warez, or kiddie porn, or all manner of other things that people use TOR for.
Assuming you definitely want to, I've tried it and it can be done. You just run TOR, and put the SOCKS proxy information into the Bitcoin client. There are several guides if you Google it.
But again, I don't think it achieves a great deal. You don't really 'log in' to the Bitcoin P2P network, you just participate in it. So unless you are planning on doing some dodgy purchases (at which point you probably need to remember that although the block chain is anonymous, it's also transparent, so any agency with an ounce of statistical hardware can just 'map' your transactions through the chain from your initial deposits to your eventual purchase), I don't think that you'll really gain anything from it. It will also waste the valuable resources of TOR on transferring the multi-megabyte block chain around.
My advice, don't bother.