Monte has two secret projects one he wrote himself which is supposedly named after a magic item in the DMG (the guys over at his boards seem to think its called the Book of Vile Darkness).And there is a second secret project written with sean K Renyalds (again the guys at his boards seem to think it in some way involves Fiend lords)If this is true it might explain why WoTC is now reconsidering.
Anyhow most of this is just rumor and inuendo but I am sure that there are two separate projects Monte said so himself on his site in an article about what he had done since leaving WoTC.
I don't think there is any doubt that the one he wrote himself will get published By WoTC it is the second one with Sean Renyolds that is in doubt.
If you look closely on Monte's boards, you can find out a lot about the Monte/SKR "secret project". Its a setting book, no fiend lords, and it was slated for mid-2003 (much more is there if you are willing to glean it).