The 512k appears to have launched for retail under the 3 for £30 offer.
http://www.home.ntl.com/page/3for30new
Terms appear to say nothing about caps if you follow the links through so I suppose you could destroy the service, if thats possible with measly 512k.
http://www.home.ntl.com/page/userpolicy
Slap me if this old news ;[