Update 2008-05-22: The real beta release has just been announced. It is the same build (804) as the beta candidate, which pretty much answers my question below on the difference between the two :).
R# 4.0 beta candidate has been released. I’m not quite sure of the difference between a beta and a beta candidate, but I’m assuming its better than a standard nightly build :)
You can get it here, build 804. I’ve had a pretty good experience with most of the "works here" builds, so am looking forward to firing this up today :)