I think CFD is a good way, because the broker don't have to have the asset, but only follow the price; what I don't know is a broker is going to risk themself allowing a long term buy on CFDs of something that acts like a commodity and is in fast expansion.
And another problem, what quote source does the broker are going to use? mtgox? btc-e? vircurex? bitstamp? another? a weighed average? That is a problem, because there is no single source of quotes.
Until the price spread become narrow enough to not allow arbitrage I think it could be a big problem the issue of the quotes.