As I suspected, Dollhouse's lower-than-expected ratings did not represent the whole picture of the show's popularity and profitability. Dollhouse has now crept to #1 in iTunes sales.
There are different possible explanations for this. One is the Whedonite masses rushing to rescue their messiah. But that doesn't really make much sense; why not just watch the show on TV in the first place?
The explanation that makes sense to me is that Whedon appeals to a kind of audience that TV networks haven't figured into their equations yet: people that are tech-savvy enough to watch a show on their own time rather than whenever the network decides to schedule it. Especially if they schedule it on Friday night, when people have better things to be doing.