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Currently (according to BaBar experts who are much further along in their VM plans) multi-core VMs don't perform nearly as well relative to single-core machines as their hardware counterparts, so the better strategy is to instantiate many single-core machines. But each one needs a license. That many rhel6 licenses would be prohibitive, so the went to Scientific Linux instead. We may want to do the same. Or things may have changed by the time it matters to us.
Experiences with VirtualBox
Installation on my rhel5 laptop was straightforward, as was creating and starting up a VM using the gui application. The only hitch was that redhat4 - actually the Scientific Linux equivalent - did not make a good guest: it failed to boot. The VirtualBox doc warns about this. Kernels which are too old have problems. In fact, they also discourage use of 2.6.18 (rhel5 and SL5 kernel) in the guest, claiming it suffers from race conditions at boot, but I haven't had any problems with it.