Kenko (parent of Tokina) has announced their first m4/3 and NEX lens offering, and guess what, it's got a mirror. The Kenko 400mm f/8 is a catadioptic lens (one that combines traditional lens elements with mirrors to collapse a telephoto optical path to a shorter length). At only 4.4" (112mm) in length, on an m4/3 camera this new lens will have an impressively narrow horizontal angle of view of 2.57 degrees (3.43 on NEX cameras).
As with most mirror lenses, it is manual focus with a fixed aperture. The lens takes 67mm filters and can focus to 45" (1.15m). That last bit puts it almost down into macro territory, as the maximum magnification is 1:2.5. No word yet on price or availability.