How I "hacked" Dustin Curtis's Posterous.
I agree with the conclusion.  Posterous could fix this problem by implementing something like The Zucchini Method (http://www.jgc.org/antispam/03152005-2150120647b00f4af9d3443... [PDF]).  Basically, they could accept posts via email as long as the user included some hard to guess word (or other token) in the subject line.