While keeping one witness for one production database instance makes sense to me, why on earth would I want one witness for multiple DB instances?
One reason could be licensing fees, even though theoratically you could use a SQL Express instance as a witness.
Remember the witness doesn't hold any data of the mirrored databases, so from that point of view there's no issue with using one witness server for prod and test. But moest customers I deal with have separate network segments for test and production servers and in that case using one witness server means opening firewall ports between the VPN's which is not really ideal.
[font="Verdana"]Markus Bohse[/font]