Nintendo's online store in Japan has begun offering a Rumble Pak for the DS Lite, model number USG-006, selling for exactly the same price as the original NTR-008 Pak — 1,200 yen.
The new Pak is compatible with the same games that supported the original. Although the DS Lite will accept the original Pak, the newly slimmed down version won't stick out of the front of the DS Lite system — though, as it is apparently only available in black, it will still visibly clash with the rest of your system if you've chosen the white or light blue colors.
No plans have yet been announced to bring the Lite-sized Rumble Pak to America or Europe. Of course, the DS Lite itself has yet to arrive in either territory; America will get Lite in white on June 11, and Europe gets both white and black on June 23.