Something that I have not seen mentioned anywhere on North American websites: Kenko has introduced a 800mm f8 DX mirror lens (supposedly in May). They are also introducing a 500mm f8 DX lens. From the July issue of Digital Photo (デジタルフォト) page 142 D.P. News. List price will be about ￥40,000 for either. Somehow, I doubt that the quality will be very good.
A similar-looking Kenko 500mm mirror lens is on auction here---while it lasts. Not sure if the 800mm is of similar in quality.