Sharp just held a press conference and announced their intent to sell ereaders and open an ebookstore. You can see their 2 models at left; specs for the 5.5" and 10" device were not released. They also mentioned that they were in discussions with Verizon to enter the US market (as well as Japan).
Curiously enough, Sharp are planning to use a custom ereader format. It sounds like they picked this over Epub because it has better video and audio support.
Update: PCWorld have more details on Sharp's ebook format. Sharp already have 29k ebooks in this format from major Japanese publishers, as well as reading apps for Windows PCs, cell phones and Sharp's own electronic dictionaries & Aquos televisions.