Thanks Goat for the response but methinks you just pulled that number from?
I remember the first Milkyway@home "Challenge" I participated in was back in Nov/Dec 2010 and lasted 5 days.
Well if I look at the historical difficulty chart I see it was 12,252 on 12/09/2010 and I distinctly remember I had (24)5870's at that time so I can use TP's calculator and plug in those numbers:
5870~400Mh/s x 24 = 9600MH/s
Back up another month and subtract (4) 5870's(I was buying one every week)
5870 x 20 = 8000MH/s
difficulty was 4536 on 11/09/2010 so we get
It would be tedious to go through that whole year plugging in the correct difficulty numbers and allow that I was buying a new 5870 every week but suffice to say I left a shit ton of BTC on that hypothetical table!