Binding M-<up> / M-<down> in Emacs 23.1.1

I always use C-h k (key) (i.e. describe-key) to find out how Emacs refers to (key), and then use (kbd) with that same string to utilise it.

In this case, describe-key returns <M-up>, so I would use (global-set-key (kbd "<M-up>") 'transpose-line-up) (exactly as J.F. Sebastian has done).

Edit:

Running emacs -nw (but not through screen), describe-key reports ESC <up> (translated from ESC M-[ A), and (kbd "ESC <up>") is successful for binding it.

Running screen emacs -nw, describe-key reports ESC <up> (translated from ESC M-O A), which seems to match what you see, and the binding for (kbd "ESC <up>") still works for me.

(n.b. Tested under Cygwin with screen 4.00.03, and Emacs 23.2.1.)

Emacs has a complex mechanism to handle the vicissitudes of function key and modifier encodings on various terminal types. It doesn't work out of the box in all cases. The following settings should work on your terminal:

(define-key input-decode-map "\e\eOA" [(meta up)])
(define-key input-decode-map "\e\eOB" [(meta down)])
(global-set-key [(meta up)] 'transpose-line-up)
(global-set-key [(meta down)] 'transpose-line-down)

You should be able to use (kbd "<M-up>") and (kbd "<M-down>") in place of [(meta up)] and [(meta down)], as long as you've done the step of telling Emacs (via input-decode-map) about the escape sequences that your terminal uses to encode these key combinations.