Thanks to Ex, a close source to Massive Attack, what appears to be the final list of vocalists that will feature on the next Massive Attack album have emerged. The vocalists will be:

Damon Albarn (Blur)
Hope Sandoval (Mazzy Star)
Tunde Adebimpe (TV On The Radio)
Martina Topley Bird (Former Tricky Singer)

Of course, in addition to the aforementioned guest vocalists, 3D, Horace Andy and (in a welcome return to all fans) Daddy G will also feature vocally on the new album, which is still yet to be named.
Guy Garvey, the lead singer of current indie favourites Elbow, has also divulged a few details on his contributions to the new album in an interview just published today by BBC6 Music. The interview reveals that even though he will not apparently be a vocalist on the album, he has helped out in the song-writing department on a total of six or so songs, one of which is Red Light (debuted live last year) which is destined to be sung by Martina Topley Bird. In the interview he describes what a pleasure it was to write Red Light for Martina. [Quote] – “It’s a dream come true to have Martina singing my words on a Massive Attack song” [BBC6 Music - July 2009]
Finally, in yet more news about the upcoming album; what exactly is the intended release date for it? An official announcement is due soon from the band on this, but speculation has it (again being confirmed by Ex), that the album will not be out in time for the Autumn tour later this year and may be ultimately pushed back to early next year. What appears to be happening though is that some sort of preview single/EP will be released in September to coincide with the tour. The band are also looking into some sort of unique internet release of some of the outtakes/b-sides from the new album which would be made free to download, in order to serve as a further preview for the album. Again, official word is due from the band on all this very soon, so keep an eye on the official site over the next few weeks, for confirmation on all this.