tardtasticx: I think its a great idea. The youth market needs something like this, but I'm not sure it will really take off. A lot of the people at my school are running on Vodafone, with Vodafone brought handsets, so the bands wont be suited to Telecoms WCDMA network, which means many will need new phones, which many cannot afford. They need to have 2G services I reckon for this to really take off.
We teens like being able to try something out, and if it doesn't work/suit our needs, we want to be able to leave and go back to where we were prior, which isnt really possible on two almost entirely different networks.
This, I believe, is exactly the reason why the phones will need to be cheap aka. subsidized and locked. Take, for example, the cheap Telecom-branded ZTE phones, they tend to retail for around $100 or so. Add in the lock and subsidy and I imagine they could probably go for about half that, maybe less? Although, like I've said, the only reason that I can justifiably see for locking a phone is to make sure that it stays on the network (which is the whole idea I guess) and to in turn make money off of the virtual monopoly you will have over peoples phones (potentially anti-competitive, in my view).