Telenet in Belgium has a Basicnet (fixed line internet) version that has a 1GB limit. I would suspect 20% of mobile users would hit that cap straight away.
640MB isn't that expensive for T-Mobile NL. Even if they had no peerings, then it would still amount to less then 3 cents a month at €5/mbit/s/month (100GB) in Amsterdam. In Australia the costs would be 60 cents/user/month. (which also shows how much money is currently made on all you can eat data plans at €9.95, which are a top up on normal subscriptions)
However if this kind of usage continues to spread to users of other mobile devices then I do suspect mobile backhaul would be in serious need of an upgrade. On the other hand a standard DSL should be able to carry the traffic