From this post:
viewtopic.php?f=1&t=20240

I would suggest setting up a script to automatically remove email addresses and convert them into something readable by people but not bots. It's easy to accidentally forget and post an email address.

-d

I'm sure that some people are helped by this, but I don't mind my email address being bot-readable. Google's spam filters do their job very well.
I guess bot makers have adopted techniques to find some hidden emails as well, so the only secure (and neat!) way would be to use images (for which bots probably don't have the computing power).

Still, this does not protect you against friends getting a virus that steals their address books.