ghci uses haskeline instead of readline. If you remapped ⌘← and ⌘→ to \e[H and \e[F, add this to ~/.haskeline:
keyseq: "\ESC[H" ctrl-a keyseq: "\ESC[F" ctrl-e
You could also remap ⌘← and ⌘→ to ⌃A and ⌃E.
See http://trac.haskell.org/haskeline/wiki/CustomKeyBindings.