I have this issue as well as a few more.
I have a mid-2010 15" MBP with one monitors; the dock only shows on what was designated as the main monitor under OS X 10.8, the center one.
Even withou the external monitor, multiple desktops don't work they way they should--at least I believe. Command+tab takes me to the window of the active but not the active app itself. I can get Command+tab to work as it used to if I have everything jammed into one desktop but that defeats the point of having multiple ones, no?
The dock does not appear on my laptop when my EM is plugged in. And the pinning options are off. The Mac will ask me when I want to tie something to an individual desktop, the numbers are way off. See here.
At least I'm not alone, I guess. Serves me right for using an early version fo the new OS.
The Dock will only move between monitors if it is set to the bottom on the monitor.
To get it to show up on the second monitor, move the cursor to the bottom of that monitor and the move down again. The dock will then disappear from the first monitor and pop up on the second monitor.
Do the same on the first, to get it back again.