My local gaming group ( http://www.rochestergamers.com ) is doing a gameday in February, and I have agreed to run a DH game. No clue about who will play, so I'm running on the assumption that players may run the gamut from zero info on WH40k all the way to tabletop veterans. Any suggestions on scenarios to run, how to introduce the setting, general demo tips etc. etc. ?

Edge of Darkness is a fantastic starter adventure. I've touted it a few dozen times before, and I'll do so again. It's got everything: investigation, combat, horror and social encounters. It's also surprisingly open eneded in how the acolytes can go about completing (or botching) their mission.

http://new.fantasyflightgames.com/ffg_content/dark-heresy/pdf/edge-of-darkness.pdf

Read through it a number of times and thought it was top notch. Think I can get through it in a 4-5 hour session? This will be my first time GMing DH.

It's possible, if everything goes quickly and the acolytes don't chase up every lead. Bear in mind that some of the nasties cause Fear (though only under certain circumstances for the more common type), which if you go will the full rules on Fear can be extremely crippling for a low ranking group. It might be worth using the stripped down Fear rules on page 25 of the PDF to keep the game running, at least until people come to grips with the system. It's also worth noting that the sample characters in the PDF have 700xp spent on them, rather than the 400xp that starting characters usually get. You may have to tweak the difficulty a little, or simply let your players start with more XP. My own group went in with 400, but they were 6 strong and half of them were Guardsmen, so even the toughest fights were managable for them.