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-nil and the current buffer is not full-frame width.
In Emacs 20, use hscroll-mode.