Well some of the following were already questioned, then just a short number of doubts are considered like "new". But anyways, that's a great thread for the curious and the not-informed people. Note that this interview is allowed by the BMS Team to be posted here. The questions have been asked from me (MenteR) to Raminator, a member of the BMS Group.
MenteR: Well first one: will xen stuff be revealed for the public? Or will we need to wait for the release of BMS?
Raminator: No, we're not releasing any media from any of the Xen chapters.
MenteR: What's the release date of the game?
Raminator: We don't know.
MenteR: 2008 maybe?
Raminator: We don't know.
MenteR: Well that's ok, do you guys have any project in mind like Black Mesa: Opposing Force, or will this be just a dream for the fans?
Raminator: No, we don't have any plans to do anything once we finish Black Mesa.
MenteR: Are you guys still accepting people in the team for web designing and stuff?
Raminator: No, we've already got our web design covered.
MenteR: And what about other jobs?
Raminator: We're currently on the lookout for level designers and character artists.
MenteR: Are you guys going to modify the G-Man?
Raminator: No, he'll be the same as he was in Half-Life 2.
MenteR: What about his voice? Same as in HL1?
Raminator: No, it would be illegal to redistribute files from Half-Life with the mod.
MenteR: Then will guys be doing your own voice acting?
Raminator: We have a team of nine talented voice actors who are bringing our dialogue to life.
MenteR: The Security Guards are going to be shown or will they be hidden just like Xen stuff?
Raminator: No, you'll be seeing our security guards before the mod is released.
MenteR: And the gun sounds? You will use some from Half-Life 2 or the whole sounds are going to be made by the team?
Raminator: All of our weapons have completely new sounds.
MenteR: Does that applies to the monsters? Headcrabs, Zombies, Vortigaunts...
Raminator: The NPCs that we're bringing across from Half-Life 2 (the headcrabs, barnacles, etc.) will use the Half-Life 2 sounds, but our new NPCs (human grunts, zombies, etc.) will use new sounds.
MenteR: In Half-Life, the Headcrab has original teeth from Gornach's Lair. Is this going to featured in BMS?
Raminator: Our headcrab is exactly the same as the headcrab in Half-Life 2.
MenteR: How much models will the common zombies have?
Raminator: There are scientist zombies, security guard zombies, and human grunt zombies.
MenteR: The Vortigaunt Slave will be the same as seen in HL2 Nova Prospekt?
Raminator: More or less.
MenteR: What about the train chapter (On a rail)? Is this going to be longer than the expected? Or shorter?
Raminator: It will be more or less the same.
MenteR: Are the Hgrunts going to be stronger than the Combine Soldiers?
Raminator: They'll be just as challenging as they were in Half-Life.
MenteR: Are we going to need to pay for BMS?
Raminator: Nothing - it's a free mod.
MenteR: How much space is it going to occupy on my HD?
Raminator: We're estimating it will be about 4-5 GB.
MenteR: Are you guys thinking about making a x360 version?
Raminator: No. The developer kits for the XBox aren't publically available, anyway.
MenteR: Is (Are) there any additional weapon(s) that wasn't included in Half-Life 1?
Raminator: No.
MenteR: For finishing it: Barney (the real one from hl2) Eli, Kleiner, Mossman and Breen, are going to be featured in BMS?
Raminator: Not per se, but there will be models that will look like younger versions of some of the more major characters.
MenteR: Thanks for the interview Raminator. Have a good luck in your project.
Raminator: Thanks.