Nintendo of Japan is now selling a "trimmed down" version of the Rumble-Pak for the DS on their website. This lies to rest a great concern shared by many that the original Rumble-Pak would not fit completely into the DS Lite’s more shallow GBA slot. Although the old "larger" version of the Rumble-Pak will fit in the DS Lite and function properly, like all GBA cartridges, it would stick out which was somewhat of an annoyance as voiced by a number of users.
The site also notes that although the old Rumble-Pak will work with the DS Lite, the new Rumble-Pak will NOT work with the old DS due to its slim nature. There is currently no word on when, if at all, the new Rumble-Pak will officially be distributed outside of Japan, but one can only assume that it will coincide with the release of the DS Lite outside of Japan in June.