Well as per thread, if DirectX plugin insists on Stereo I would expect the interleave button to be greyed out and stuck in stereo (so we are not fooled into thinking it is in mono if it is placed in mono, as is presently the case), or (better still) just make darn sure the output is mono.

I should be able to quickly glance across the channels of a console and not have to worry about whether Sonar is lying to me about stereo/mono outputs.

Thank you everyone for the replies.  I was totally lost last night but looked at it again with fresh eyes tonight.  I figured out that I had replaced the Sonitus fx compressor on the master bus with the Cakewalk Stereo Compressor Gate.  That was forcing the Interleave to stereo.  The Plugins both say "Compressor" in the bin so I didn't immediately recognize that it was a different compressor.  
I will however give the Channel tools a try for added control.   I am learning a ton just from trial and error but its nice to know there is a community out here when I am really in the tall grass.

CakeAlexS
if DirectX plugin insists on Stereo I would expect the interleave button to be greyed out and stuck in stereo (so we are not fooled into thinking it is in mono if it is placed in mono, as is presently the case)

 
Is that really "presently the case"? In X3 if the interleave button is set to mono, it shows mono and the audio is mono. If the interleave button is stereo, it shows stereo and the audio is stereo. I have yet to find a plug-in, DirectX or otherwise, that defeats that. (There is the POD Farm weirdness, but that has nothing to do with the OP.)

I should be able to quickly glance across the channels of a console and not have to worry about whether Sonar is lying to me about stereo/mono outputs.

If you place the old DirectX Compressor/Gate in the Master bus, the stereo interleave works perfectly both functionally and in terms of the graphic representation. Same is true in tracks. Perhaps the issue is that the OP is using X2 instead of X3. I can't reproduce the OP's issue, so if this indeed an issue in X3, perhaos you can furnish the steps needed to reproduce.
 
In any event I doubt Cakewalk would take the time to write a patch for X2 that grays out the interleave button to accommodate certain anomalous situations involving DirectX plug-ins. X2 is from a different time and a different ownership.

Yes it is really presently the case in X3E and confirmed by Cakewalk. Full steps to repro are in post #4 of the link I gave in my first post in this thread. Thanks Craig.

I'm confused. Is that the problem the OP is describing? I did what he did, i.e., put the Compressor/Gate in the master bus, and it wasn't stuck in stereo. I could set the interleave to mono or stereo, and the track played back as the interleave button indicated. My understanding was that he couldn't get out of stereo, but I didn't have any problem doing that. So I can't reproduce his problem, and the steps you referenced don't seem to reproduce the problem he's experiencing, either.

Just for Clarification.  The plugin I added was a Stereo Compressor.  As soon as I removed the stereo compressor I was able to change the Interleave button back to mono.

And to be clear at my end, this appears related to the the first post (#1) in the thread I quoted. So you might conclude the behaviour in X2 had not changed with any of the Sonar releases afterwards until the X3E patch was introduced.
 
Since the X3E patch was introduced the behaviour changed (see post #4 of that same thread for current behaviour).
 
Just to emphasise again the OP was using Sonar X2.

bellownc
Just for Clarification.  The plugin I added was a Stereo Compressor.  As soon as I removed the stereo compressor I was able to change the Interleave button back to mono.

Please tell us which compressor you were using.

Curious why you would want the Master in Mono.  Not many people make mono products anymore.

Ultimately if something is played back over speakers in a room, there will be a mixing of the two channels in the air itself. Rooms are bad enough without adding more problems...