Stupid stupid registration question

[ZiNgA BuRgA]

reCaptcha should be good.

I'm somewhat doubting bots actually do crack MyBB's system, due to its lack of popularity, but there could be.  Most spammers these days are human anyway.
Something like DeCaptcher (uses cheap labour from 3rd world countries to do their bidding) will bypass any captcha system you have anyway.