Voigtlander is offering lens adapters (available shortly) that allow the use of Leica M lenses, Nikon F lenses (with an aperture ring) to be used on the small-body-big-sensor Sony NEX-5 and NEX-3. See my lens adapter page for other adapters.
The Sony NEX-5 looks like an interesting new camera, the start of a trend, and for that reason I’ll probably review it, along with some Leica M lenses.
I expect a new crop of these large-sensor-small-body cameras to appear in 2010/2011, which means that your investment in Nikon lenses (with an aperture ring) or Leica M lenses is something that increases in value. It also means that Zeiss lenses for Nikon are a great choice, because with an adapter, you can use them on Canon EOS or Sony NEX (also Leica M9, but no way to focus makes them pretty useless until a Live View M camera appears someday).
Canon users are out of luck, since EF lenses have no means to control the aperture. The same is true of the latest Nikon “G” lenses, a disturbing trend.
You can get these adapters at CameraQuest.com.