The following is thanks to Lord Icklebum who had the pleasure of speaking personally with one Dan Abnett. I'm personally looking forward to some of these.

I sat in on the Black Library Seminar at GD Chicago, and then afterward got to have a nice chat with none other than Dan Abnett himself.

Here's what I gleaned for 40k:

The next Horus books will be Descent of Angels, and then Legion by Abnett. This has been mentioned before, but Legion will be about the Alpha Legion and Dan was quite excited about tackling them because, as he put it, no one really knows anything about them.
He stated that at least 4 major secrets will be revealed in the course of the book, some of them quote: "Forever changing established continuity"

The next Gaunt's Ghosts book will be "Only in Death" which will be his last for at least a year. It's the last in the current arc (consisting of Traitor General, His Last Command, and The Armour of Contempt), and there are apparently going to be some shocking deaths in it.
However, he said he does have the next book title in mind, which is "The Enemy of My Enemy".

He mentioned that there will be another Trilogy spinning out of the Ravenor/Eisenhorn books called either the "Kiss Trilogy" or the "Nail Trilogy" following other members of their retinues. He said Eisenhorn will be back in at least a supporting role, and there definitely will be at least one more true Eisenhorn novel.

He has a sequel for Double Eagle in the works called "Interceptor City".

His next major project will be "Titanicus". Which is, as he put it, the 40k version of Das Boot, following the crew of a Titan fighting against an enemy he "hasn't used before". This is supposed to be a full sized novel and will be runnin something like 500 pages.

When asked about Brothers of the Snake, he said that he hadn't intended on writing a sequel, but was asked so much about it at the convention that now he's seriously considering it.
I also asked him if he knew who the Iron Snakes' primogenitor chapter was. He says that initial he thought it was just going to be the Ultramarines, but the more he thought about it the more that it seemed kind of bland. He winked and said he'll have to think about it and perhaps reveal it in the future.

In non-Abnett related rumors:

There's a new edition of the Uplifting Primer coming out called the "Damocles Gulf" edition. This is updated and expanded.

Graham MacNeill's also writing a companion primer "Munitorium Manual" which goes further in depth on the weaponry of the IG.

There's a Sandy Mitchell penned Inquisitor novel coming out in the Fall called "Scourge the Heretic"

The Warhammer 40k RPG is now slated for February '08.