For the life of me, I could not figure out why I was only getting audio out of my right MacBookPro speaker. Live chatter Eric Wrixon made the reason abundandly clear:

I think I might know why your Skype client on your Macbook Pro only plays audio out of the right speaker. If I’m not mistaken the microphone on the MBP is located under the left speaker grille. If the audio in your Skype conversation was played back through the left speaker it would generate a lot of nasty feedback into the mic. The client may be purposely using only the right speaker to reduce the amount of microphone feedback. I don’t know if that really is the reason for the Mac client using only the right speaker, but it sounds like a reasonable explanation to me.

Makes total sense to me now.