Some answers:
1) What has become of the "D" Series? I likes "Tomb of Abysthor" a lot, but will this be all there is?
Our quaint lettering was a throwback to the old school modules, but, like the old school modules, doesnt really have a good rhyme or reason. I'll write more on the lettering in a bit.
2) Would it be possible to get more accurate information about the Judges Guild schedule and plans?
Yes. City State of the Invincible Overlord just got advanced print copies back from the printer yesterday and looks unbelievable. It is available for preorder now and should be in stores by November at the latest.
Caverns of Thracia was available at GenCon and should be in stores soon if not there already.
The massive 18 map, two booklet boxed set of the Wilderlands Campaign Setting with cover art by Frank Frazetta will be out next GenCon.
We also have plans to release Tegel Manor next year near the end of the year.
3) As Necromancer Games seems to be getting more and more into "Mini Campaigns" (that actually rocked, I´ve been thru VoLK, Grey Citadel, and Durbenford lately), is there any chance to see a follow-up campaign for an existing one? Any chance for a "Return to the vault of Larin Karr" or anything like that?
Probably not. "Return to"s generally dont work unless lots of time has passed or they are out of print. Rappan Athuk Reloaded will work because it compiles everything into one volume and is totally redone. With new extra levels and stuff. But I dont see a return to VoLK.
4) DM-Series. Is it dead? Does it stay dead?
You never know

We have changed around some of the lettering. DM series was to help DMs. We have some sourcebooks coming up, like one on forest adventures called Glades of Death that could well be considered DM series. But we are kind of moving away from being restricted by the "letter" system.
5) What ever happened to the open call on your forum for the 10 module mega-adventure? I know you received submissions but there hasn't been so much as a "we're busy right now, but rest assured this project will be discussed soon" message. Just curious.
We got some really good submissions. As you can imagine, those are long in the writing. So we have several in development. But since they are open calls, you get people who may not have a ton of publishing experience. So you may get half a manuscript and nothing more. So we are waiting until they get closer to announce them. But we got some good stuff.
Clark