Ok... I went on my husband's computer... he doesn't have my powers... /ubbthreads/images/graemlins/grinser2.gif ... and I found that the number of invisible users online IS reflected in the numbers shown at the bottom of the Main Index page. And what's really kind of odd is that it is reflected at the top of the list on the "Who's Online" page as well. But if you actually count the number of registered users online, it will not match what the number is at the top of the list or on the front page.
So in conclusion... /ubbthreads/images/graemlins/wink.gif ... The numbers indicated are correct and include every user browsing the board at that time... invisible or not. But the "Who's Online" page only shows users who are not in invisible mode.