The problem with only CVport is it equals 2 while on the Model tab (i.e. when TileMode=1), don't know why - I'd have imagined 0 would be more useful. Though if you have tiled viewports on the model tab I could possibly understand. But a CVPort=2 thus might mean you're inside the 1st viewport on paper space, or it might mean you're on the Model tab.
Edit: Actually, I understand what you're saying Lee. If all you really need to know is which space is active, then CVPort>1 means Model Space is active, CVPort=1 means Paper Space.