VritualBox supports OS X guests inofficially and only on OS X hosts. But the EULA states:
A. Single Use License. Subject to the
terms and conditions of this License,
unless you have purchased a Family
Pack or Upgrade license for the Apple
Software, you are granted a limited
non-exclusive license to install, use
and run one (1) copy of the Apple
Software on a single Apple-branded
computer at a time. You agree not to
install, use or run the Apple Software
on any non-Apple-branded computer, or
to enable others to do so. This
License does not allow the Apple
Software to exist on more than one
computer at a time, and you may not
make the Apple Software available over
a network where it could be used by
multiple computers at the same time.
I tested it: Install Virtualbox, insert your disc and create a new VM - works like a charm!
 i changed my answer do reflect the actual EULA terms (which still don't apply in some countries like Germany)
Here are some screenshots:
The first one shows Snow Leopard (MacBook Installation DVD) in VirtualBox 3.2-BETA1. The second one shows the same virtual machine (created in BETA1) running under 3.2.8 and booting from a Leopard Upgrade DVD.
Creating an OS X Server VM (without altering any other settings!) worked fine for me, too!
I didn't complete the install, though. My 10.5 Insallation DVD requires 10.4 to be installed and i didn't have enough time to install 10.6 either.