metronet is tripling its allowances, there is still no hard cap but the allowance on the minimum price is triped so eg. 2mbit 20:1 adsl for includes 150gig traffic at £27.99 a month just £3 more then ntl with a extra 100 gig traffic and many other inclusive services.
http://www.adslguide.org.uk/newsarchive.asp?item=2326
Also wannadoo a major player in the market is trialing its LLU service, if the isp moves over to LLU I would expect an increase in usage allowances which would probably force both BT and NTL's hand.