As far as I know, as long as all the forums use the same version of the software, you can set them up to do that; however, the cookies are going to be a problem. You'd have to hack the source to use a Single Sign on type of system so that once you register at site A you're recognized at site B and site C and won't register again.
No forum software that I know of immediately supports this; however, as I said, a bit of hacking could get it to work. I know SMF has had some people use a generic users table to have the same users across multiple forums; but, I'm not sure how successful or easy it was.
