Bandwidth limits are a growing an inevitability with ISPs; NTL introduced them a while ago, and now BT appears to be doing the same thing. BT Broadband, who have seen massive customer growth in the past year, are targeting high network users, in an attempt to reduce pressure on their network. The new terms and conditions say this will affect only a small minority of users.
Each month, BT customers will get 15/30 gigabytes (512k/1meg respectively) worth of bandwidth. If they go over that, they will be charged for extra bandwidth by the gigabyte (£2/Gig). BT claims, correctly, that introducing measures like this will improve their offerings for most customers. And to be fair to BT, this will affect few people; "mom and pop" users would never (ok, find it hard to) approach 15 gig of traffic a month. BT is targeting users that download movies, download large files, and generally put high strain on the network.
It would appear that BT customers have little choice in the deal, asides from voting with their wallets (and moving elsewhere). The only other major broadband provider that doesn"t have these restrictions is Blueyonder.