It seems that universities are responsible for some pretty good acapella groups - and some of these groups love to sing Jamiroquai.  In April there was a news item about a group from University of Massachusetts Amherst who have done a great version of Runaway.

I've just been told about a group called Out Of The Blue from Oxford University in England who have covered a couple of Jamiroquai tracks over the years, and if you visit their site at MySpace you can hear their version of Virtual Insanity.