A plugin would be welcome. I doubt anything will be added to the core.
How about a different direction, ad an activation code in the e-mail while registrating. The password is already e-mailed, so put in an activation link aswell. If not activated using the link in 1 or 2 days, the person wont be registrated at all.
Many forum software have something like this, why not bbpress?
Because it hasn’t been developed, of course.
Either write a plugin or write a patch and propose that it be added to core.
Why would I want to stop spam registration? That is how I am increasing my userbase! (can’t you just taste the sarcasm) 
Trent
To prevent the registration in the first place, why not a CAPTCHA?
http://recaptcha.net/index.html
It could probably be added to a bbPress install with a plugin. Or, a while ago, Trent mentioned he added a couple of fields to his registration form, and the bots haven’t figured that out yet.
Also, it seems like the bots all tend to fill out the interests and website, but users normally don’t. Maybe you could add something to your template that says “Please don’t fill out this field” that a bot could not read, would fill out, and then you could write a plugin based on that?
Think I am going to try something like the in e-mail activation link. Think that is the best approach…
