After getting most of my problems solved I decided to move my test 
environment onto the production server.

The server is a dual xeon which, with hyperthreading, appears (to Linux) 
to have 4 processors.  So, when I built this machine I decided to use 
the ovzkernel-2.6.18-smp

The rebuild caused me all sorts of routing problems which I have managed 
to track down to being caused by the kernel.  I just replaced the kernel 
with ovzkernel-2.6.18 

aptitude install ovzkernel-2.6.18
aptitude remove ovzkernel-2.6.18-smp
shutdown -r now

problem solvered!

It seems pretty odd that the smp kernel sould cause this, but I really 
don't know what is different about that kernel.

The symptoms were similar to the ones I had before I set the netmask of 
the venets correctly, but more extreme.  Whereas the netmask issue 
seemed to cause packets to go out of the wrong interface, this problem 
seemed to stop packets getting out of the server at all.

If there are any questions about the symptoms, I will be able to swap 
back to that kernel for the next day or so to test things out.

What will the impact be of running the non-smp kernel on a 
multi-processir machine?  Will I only effectively use a single processor?

Steve