In reply to RyanC: I'm sorry to hear about your frustration and hope you can take your 'newbies' on the wall soon despite this.
Like yourself I've had no issue with other walls as I've been allowed to sign in as the supervisor for complete newbies, or inexperienced climbers (Awesome Walls, MCC, NorthWestFace and even Rock Over). Only Awesome Walls took issue with one of my beginners as we didn't have a third person present to assist with belay technique should I be up the wall (a common sense attitude).
Of all the times I hit indoor walls I've only ever had a beginner stopped at MCC and even then, only once. The reception staff were insisting that because he was under 18 he needed to do the intro course before he could be signed in with a supervisor despite the fact he'd been with me to MCC before. Safety and conduct were the reasons given but we eventually got the situation resolved and got in.
The amount of questioning or the policy on newbies seems to be very much dictated by the gut-reaction of the staff on-duty. I can honestly say I've never been treated unreasonably and have always had some level of questioning when I sign in a beginner but by the same token it's not been a consistant thing - some staff go through your responsibilities more thoroughly than others. I guess the same is true when making a call on whether a beginner needs centre-training or not before entry.