I just put together this spreadsheet this week.... Maybe other people will find it useful. I'd love feedback if folks have other data, corrections, alternate interpretations, etc:
https://docs.google.com/spreadsheet/ccc?key=0Am5Tfupsz5CfdGppUUVxLWttVzZzSHpxYzNPWjVDYUE#gid=5I tried to use whatever data I could find and then just make guesses to project forward. I tried to make be as conservative as possible, where conservative means the hashrate is going way up, because I'm trying to estimate whether I'll recoup my investment in a BFL single.
Doesn't look that good for me, but I still have hope.
Worst case scenario I'll help decentralize the network a touch.
Edit: Sorry, pasted the wrong url at first!