When thinking about bitcoin one must remember it is divisible to 8 decimal places. That's a lot of coin. Maybe even enough to justify mobile mining at some point.
I've done a lot of thinking about this since I started my little 5gh/s operation 3 months ago. I found that the best return was to start on used 5970's and use the btc to bump up to 6900 series that have plenty of warranty left, at this point I still want my mining rigs to be relevant to gamers who are looking for used gpu's in case gpu mining becomes obsolete. Not the best efficiency, but my power is cheap and I have a cold basement lol. That said, anyone want to buy a watercooled 5970? It has an EK plexi top block on it, PM me for details.