Single Signin accross WordPress, WordPress MU and BBpress?

Hi, I’m working on a site idea and would like to have the following set up:

There parts,

wpMU.domain.com
bbPress.domain.com
wp.domain.com

Is it possible for a user to sign up for one part of the site and their credentials work for all the three applications?

For example, a user sign sup for the WordPress and is able to use the same log in information on the other two sites, WordPress MU and BBpress?

If so, how do i go about setting that up?

Thanks, Ross.