The Nintendo DS Rumble Pak, packaged with 2005's Metroid Prime Pinball in North America, is going to be sold separately in Japan at a cost of 1,200 yen (about US$10) through Nintendo's online store in that territory, according to Famitsu. The Rumble Pak is also sold in Nintendo of America's online store for $9.99 as well as continuing to be bundled with Pinball.
Famitsu also notes that the Pak is compatible with just three titles thus far: Metroid Prime Pinball, Mario & Luigi RPG 2 (known stateside as Partners in Time), and Puzzloop (which we'll see as Magnetica come this June). We can also confirm that the Pak works with Super Princess Peach as released stateside, but we don't know whether that was the case in Japan.
Thanks to Samer Abbas for the tip.