I would look at BTC/GH/s per $ invested.
As your projections show, BTC/GH/s is falling rapidly (nice blog BTW) but when you factor in CAPEX, the numbers look more mundane.
Now, to know whether you'll be making money in $ terms is another issue (as always, it depends on the market). My guesstimate is that the BTC should hover in the 50-70 USD range when all the crazyness has dissipated but things are very much in flux right now so I wouldn't bet on it
At 50 USD/BTC, you're looking at a profit of circa 20 USD/day in August for 50GH/s (0.008 BTC/GH/s per day), not factoring power consumption. If you paid 50K for your ASIC (and got it in August), you just lost a ton of money. If you paid 2.5K, you'll get the same ROI as ppl used to get on GPUs when spending the same 2.5K on a PC.
I would plug in some CAPEX numbers in the tables/graphs to have a guesstimate as to how much one can invest and when, if one wants to make money (but it looks like you're doing it much better already :p).
I've included exchange rate in previous posts in the series, and produced tile plots of exchange rate / ROI with days to ROI as the tile colour. It's interesting, but at the moment everyone seems to want their estimates based in BTC, and no one seemed to know how to interpret the charts.
I agree it's an important factor, but back in January I'd only included exchange rates up to $40 and even then people though I was being optimistic. Point is, BTC market is too volatile to use an estimate - you have to produce a large number of estimates for a very wide range. If readers want that back, I'll bring it back next time.