more info
 

Thanks to all for helping.

I will follow these suggestions.

One thing I discovered is if I create a simple cube in the same scene and hide it, then select my head and mirror it - I can see the option in channel box to play with the tolerance. Funny behavior indeed.

I also wanted to say I am really thankful that folks replied to my post - I recently found simplymaya and now I am hooked.

Great!
dan

Solution
 

I was having the same problem, Daniel, and I was looking at the replys here.
I think the:
duplicate,
negative scale the correct axis,
mesh>combine mesh (in object mode with both sides selected),
and then a massive edit mesh>merge vertex fiesta would essentially work

buuutttt.......

I got frustrated, saved, closed maya, reopened, loaded my scene, and tried the mirror geometry again
...
and what do you know, it worked. ;)

I hope this helps...

ALSO, I found this also solved a problem I was having where hitting f wouldn't focus the view

Mirror issue
 

Hi, you want to go to ChannelBox -> Inputs -> polyMirror -> Merge Threshold = .001

That fixed my problem

Good luck!