I'm currently investigating the same question. I have cash in a Roth IRA (have not withdrawn it) and I've asked the manager of my account about bitcoin. Unfortunately, he can't help me. Basically, no one where he works knows jack shit about bitcoin beyond what I've told them. He says that I need to find an IRA custodian who will allow holding bitcoin in their IRA.
From my internet research, I think there are two options:
1) set up a self-directed Roth IRA, set up an LLC, and buy bitcoin through the LLC. I don't know enough about this to know how to do it or what pitfalls may be involved, or even if it's doable.
2) Do as advised above: find an IRA custodian who will allow holding bitcoin in their IRA.
...
So far that's all I know. I'd be interested in hearing from anyone who has actually done this!!!
Back at Christmas 2013, I was where you are now. I realized that a Roth IRA was the best place fom my bitcoins, as they can grow tax-free. I hunted and hunted, and eventually came to realize that the IRA LLC was the only solution available to me. (I'm not rich enough to invest in the SecondMarket BIT.) I wrote up what I had learned here:
https://bitcointalksearch.org/topic/how-and-why-to-hold-bitcoins-in-your-roth-ira-yes-you-can-do-it-today-396783.
As far as I know, this is still the only way for the average Joe to own bitcoins in an IRA.
It took about a month and cost a couple thousand dollars to set up and fund the IRA LLC. It took me another 6 weeks to purchase the bitcoins for my IRA LLC, partly because I decided to exclusively use U.S.-based exchanges. If anything went wrong, at least I could sue them in a U.S. Court.
CampBX turned out to be too difficult to fund, so I wound up buying them all through Coinbase. Because of their buying limits, I had to make 50 separate purchases over a 6-week period.
In all it was probably more than 100 hours of research, filling out forms, talking to people on the phone, emailing, overnighting, waiting, roadblocks, frustration, watching prices, limit orders, and endless accounting. But with the help of a strong stubborn streak, I succeeded. I now have a nice little nest egg of bitcoins in an IRA LLC owned by my Roth IRA.
Now I'm just waiting for the price to go up!
It's not easy, fast, or cheap, but it's doable.