PHP/sql/MTA solution (Score 1)

How about this approach, which would not require anything special on the client side?

PHP (or another suitable server-side dynamic page generator) would append strings to the email address in question before placing it on the page, (strings built from a function known only to the page author or website admin), resulting in a unique mailto address for every page load. Simultaneously the function logs the unique address in a database (e.g. mysql), with a timestamp.

So the resulting HTML might look like:

href="mailto:[e-mail]"

The MTA (e.g. qmail) is extended so that it examines all mail coming into the domain. If the addressee has a string on the end that fits the function, the extension checks the database for that entry. That entry is only allowed to pass 1 email through. If no email has passed through yet, and the address was "recently" registered in the db, the MTA extension marks address is "used up" and records who it's from. Then the extension passes the email through to the intended recipient, in this example "[e-mail]". If the address already was used to pass an email through, or it was registered "a long time ago" (e.g. more than an hour after the page was generated for a browser), then the MTA extension would reject the email.

This way, if a harvester gathers the address in a crawling sweep, it would have to immediately fire off a spam email to that address in order to take advantage of it (unlikely since my understanding is that harvesting happens separately from spam runs). Even if spammers re-tooled to do "collect and spam immediately", they could only send one spam per harvest, and not resell or reuse the email address later since it's only good for one shot (like many anonymous address offerings out there).

I haven't seen anything quite like this implemented but would love to know if someone's already offering/doing this.