A quick note to Hyper-V users. When I installed Windows Server 2008 Service Pack 2, it installed fine with no errors, but after a while I was getting NetBT errors in Event Log saying there was a duplicate name on the network, and other issues finding machines on the network.
Turns out that the service pack had re-enabled all of the network connections in the base O/S running Hyper-V. As two of these need to be disabled (see http://blogs.msdn.com/alexhomer/archive/2009/02/08/Hyper_2D00_Ventilation_2C00_-Act-III.aspx for an explanation), this meant the base O/S had two connections to the internal network, one of which was obtaining a new IP address through DHCP and registering itself in DNS.
After disabling these connections again, everything returned to normal. Maybe I should have deleted the connections in the first place instead of just disabling them... any advice from an expert in this area would be very welcome.
Update: In Windows Server 2008 R2 you can untick the Allow management operating system to share this network adapter option in Virtual Network Manager to remove these duplicated connections from the base O/S so that updates and patches applied in the future do not re-enable them.