How do I have two copies of iTunes using the same library?
Is iTunes Match an option?  I have a a Mac Pro that has my actual iTunes library, but I have full access to it from my iPhone, MBP, AppleTV, etc.

It's not free, but it's a pretty significant service IMHO.  I think it's probably the most elegant solution :)

Ok, so your best bet is probably going to be the external HD.  


Make sure iTunes is closed on the iMac.
With your external HD plugged into your iMac, move the entire iTunes folder (~/Music/iTunes) to wherever you want on the external
Start iTunes while holding down the option key.  This will allow you to tell iTunes where to look for the iTunes Library.  Choose the iTunes folder you just moved to the external.
Quit iTunes and eject the external from the iMac
Plug the external into the MBP and start up iTunes, again, holding down the option key to then point the MBP to the Library on the external.


If you want to keep the iTunes library in sync between the two computers, you'll need to have that external plugged in and moving between the two every time.  

If you're not terribly concerned with keeping it in sync all the time, you can skip the step above that tells the iMac to look in a new place.  Just leave the iMac alone and point the MBP to the copy on the external.  You could overwrite the external once a month or something.