flower, I realise that it isn't simple and that a TA from a TCAS could cause some chaos, but I'm not talking about a situation where I'd make that climb just for fun. I realise that they are different controllers too. But, and perhaps AlanM could comment on this, is the entire system really that tight - if so surely they'd be fairly regular problems with inexperienced pilots climbing a few hundred feet in to CAS? Or a dicky altimeter? The wrong QNH? The way you describe it does make it sound as though as soon as you go over 2500 you get hit, directly and without question. Do you see what I mean? I mean, I realise we'd be talking about reduced separation, avoidance moves, delays and god knows what else, but isn't that better than crashing your plane in to a bus stop full of people. All of which makes me think that perhaps the whole transit idea is a bad one. Back to square one!
I'd really like to hear some other opinions on this particular aspect of it.