I think ntl have got it right, 1mb is an entry product and should be capped that way, the other tiers look way more prospective then it so maybe theres that extra incentive to for a couple of customers to bump up their speed.
Actually, I've got to hand it to them, as long as they keep the guidelines for the upper tiers, it makes all the mainstream and alot of the more specialist isps, such as Pipex look weak in comparison