Is there a keyboard shortcut to queue a selected song so that iTunes will play it next? Is there a keyboard shortcut to queue up a selected song to the "Up Next" playlist that was introduced in iTunes 11? 

I like that new feature but having to alt-click or select from a panel isn't fast enough to queue up a few songs rapidly.
optionenter (shiftenter on Windows) will add the selected song to the Up Next list (in iTunes 11.0, this does not work in album view, but does in all other views). However, iTunes will always ask you if you want to clear the previous songs from the list, so an extra keystroke is needed.

There are one or two drawbacks to this method, firstly: It defaults to clear the previous songs so a simple enter will not suffice and a P keystroke is needed to add the song while keeping the existing list. Secondly: In addition the added song will default to the top of the 'Up Next' playlist as oppose to when you click to add a song (in this instance the added song will default to the bottom of the list).

I'm only running Windows so I can't say the same (or similar) is true for Macs.