Orlando Bloom hasn't officially signed on at this time, although he is closing in on a deal to return. Orlando Bloom appeared in all three Lord of the Rings movies - The Lord of the Rings: Fellowship of the Ring, The Lord of The Rings: The Two Towers and The Lord Of The Rings: The Return of The King - as the skilled archer elf Legolas. Although the character doesn't appear in The Hobbit novel by J.R.R. Tolkien, the role was said to be larger than a mere cameo appearance.
It has been speculated for some time now that other original Lord of the Rings cast members such as Ian McKellen (Gandalf), Andy Serkis (Gollum) and Hugo Weaving (Elrond) will return for The Hobbit, although those deals have not closed yet.
If Orlando Bloom does sign on, he will join another original cast member, Cate Blanchett, who we reported just yesterday will return as Galadriel, another Lord of the Rings character who does not appear in The Hobbit book.