Kenko today announced two new DG extension tubes for mirrorless users: 10mm and 16mm tubes that fully support the electronic signals from the lens to the camera (including autofocus). The tubes will be available in m4/3 and NEX mounts.
An extension tube mounts between a lens and the camera body, has no optics, and essentially produces an effect similar to what would happen if you were able to continue extending the focus mechanism in the lens past the near focus point (further away from the camera). The more extension, the closer the focus. You do lose infinity focus when you use an extension tube, so they're mainly used by macro enthusiasts.
Note that you can stack the tubes to get 26mm extension. However, there are some caveats you need to be aware of. Generally you shouldn't use an extension tube longer than the focal length of the lens (it's possible that you extend focus into the lens itself with some designs and you certainly would have little or no working distance to the subject). At 10mm, 16mm, and 26mm stacked, you'll have limited choices when picking m4/3 lenses. Very wide angle lenses will be a problem even with the 10mm, for instance. Moreover, Kenko notes that you can't use the two tubes stacked on the Panasonic 45mm Macro lens or the 45-175mm power zoom lens.
Similar tubes have appeared earlier, but these are the first I know of that support the lens electronics (autofocus, exposure, EXIF data, lens distortion information, etc.) that have shipped for m4/3 and NEX.
Apparently the tubes have already been released, as there are eBay auctions on them that have appeared today. Here in the US, Kenko is distributed by Kenko Tokina USA, so dealers, especially big ones like B&H should be getting them shortly.