This page used to describe how to build a now-outdated LinuxCNC branch for a variety of kernels on Debian Wheezy.
To install a copy of Debian Wheezy with an "RTAI" realtime kernel, please follow the instructions here: http://article.gmane.org/gmane.linux.distributions.emc.user/52401
This comes with the latest-at-time-of-writing 2.6 prerelease. You can use master branch by enabling buildbot packages or by building them yourself.
If you are interested in the experimental "RT-PREEMPT" realtime on Debian Wheezy that has been merged with the LinuxCNC master branch, please follow the instructions on the wiki page Uspace.