Hashcash is (imho) a stupid waste of time. It unfairly penalises people with slow computers who might want to run a mailing list (me!) while barely slowing any spammer who happens to control a vast network of virus-infected home computers (most of them). And, like most spam 'solutions' it requires a complete reimplimentation of the entire mailsystem.
What? Please read about it first. In the case of a mailing list, the mailing list computer itself doesn't need to do much extra work, it certainly doesn't need to generate hashcash headers. It doesn't require a complete reimplementation at all - it is a plugin in your mail client and a plugin in the ISP's MTA. The hashcash computing should be done by the user sending the mail. Only verification (which is apparently VERY quick) is done on the server side of things. Having said all of this, I still don't see it as a very good option myself. But that is just my opinion...