I had the same problem and just solved it. For me, it was not libncurses -- I tried libncurses from Mac OS X 10.6.2 and I also tried GNU ncurses (compiled GNU ncurses and subsequently and re-compiled and linked Midnight Commander (4.7.1) against the GNU ncurses). Both attempt were futile.
I even changed the Terminal.app, which did not solve my problem either.
Finally, I simply removed the preferences file for Terminal.app. Quit Terminal.app, delete ~/Library/Preferences/com.apple.Terminal.plist, launch Terminal.app and start Midnight Commander. For me, this worked like a charm :-)
I tracked the problem down a bit further. In Terminal's preferences I usually make a copy of the "Pro" settings and make two modifications to "Pro Copy": I set the background to solid black (instead of black transparent) and I want the window to close when the shell exits. Whenever I make these two changes, the cursor keys become incredibly slow in Midnight Commander. As long as I use the standard "Pro" settings, MC works fine.