It is an intriguing idea. I only have a tiny amount of BTC, so I bought 80MH on Cex: in addition to the 330MH of my little Block Erupter, I'm positively rolling in satoshis.

It does seem, however, that you can roll your mining output into the purchase of more hashing power on Cex, so I might do that. 0.1GH, here I come...