In Emacs 21 and later, this is on by default: if the variable
truncate-lines is non-
nil in the current buffer, Emacs
automatically scrolls the display horizontally when point moves off the
left or right edge of the window.
Note that this is overridden by the variable
truncate-partial-width-windows if that variable is non-
and the current buffer is not full-frame width.
In Emacs 20, use