Oh those boolean things. Follow the steps mentioned by VelikM. I would advise you to use 2.25 instead of 2.24 as the number of polygons in the output is really quite phenomenal in 2.24 (If I remember the versions correctly). Hopefully there will be a better version availble when the sources are opened.
Occasionally you might get a message ‘Sorry - internal error’. Sometimes you can get round it by just doing the same operation again. Otherwise try nudging one of the objects.
Also don’t expect it to work well when either of the operand meshes is not closed. Coincedent faces are also not handled too gracefully.
I guess this brings up another interesting question. If blender goes open source this presumable means we can get access to a lot of this type of common code that has been written in other open source projects. It also makes a lot of sense to use a seperate open source version and put the effort into improving that instead of reinventing the wheel as so many commercial projects are forced to do. Prime examples of this type of open source functionality already written are
Booleans, IK, Decimation, Physics, collision detection, subsurfs.
What else do you think/know is available in the open source domain for us to use in free blender?
Maybe this discussion should be transfered to a different forum.