An old thread i know but there is one point I think that is worth considering is if the box connected is configured as either a dNS server or DHCP server and just so happens to have the same address. That can then cause problems.
I have already had the experiance where some very intelligent engineers set up an "isolated lab" with a full PDC,DHCP ,WINS windows domain infrastructure. They used the private address range as normal and copied the exact addresses used on the production network. No problem till one of these very smart people needed to down load some patches from the net so he plugged his lab into the production network. You can imagine the mess.