This was an issue under 9.3.1 also.
If you create an MXD with two layers that have use the same data source and then try to create a map service from that with the WFS enabled...
The service will be created, but it will not start until you uncheck the WFS support in the service.
The error message returned is: Server object instance creation failed on all SOC machines.
If this support is not going to be enabled, can the message be fixed to be informative?
