So, I just did a quick reality-check to see how feasible mining would be.
If you have a 1GH/s machine, then when the network is running at 50GH/s you control 2% of the network. If block time is 10s then you will get about 170 (out of 8640) per day. With a block reward of 5 Eth and ignoring gas (and other complexities such as uncles), that's on the order of 1,000 ETH per day. At the pre-sale rate, that's 0.5BTC.
Obviously this is a very rough calculation.
I would like to ask those of you who are actually building dedicated mining rigs -- what other factors do you include into your calculations to decide how much to invest?