Once miners like myself grow large enough to be able to buy electricity on half hourly auctions and mining at night and/or not mining at peak hours the hobbyist are going to be driven out of the mining market and I bet it will happen by 2012-2013.
There are some singular miners that you can't undercut. I don't really understand why I have to keep pointing this out, but much mining occurs in the tiny efficienty type flats of young gaming geeks. Who live in tiny flats with only electro-resistive heating, and would buy an high end graphics card
anyway, most of whom already had one before learning anything about Bitcoin. This may not be
your situation, but there are certainly people who do live in such a situation, and there is no professional mining setup that can drive these guys out of the mining business. I'd have thought that the market experiences with GNU/Linux and other open source software would have long proven that for-profit companies cannot compete with geeks who don't expect to make a profit.
And when the heat demand for those guys drops off, then the heat demand for their peers in the Southern Hemisphere will go up.