Vodafone did offer 1 GB download without time limit. However they now seem to have withdrawn this and come into line with the other operators who offer time limited but contract free. Even then with Vodafone you had to connect at least once every 180 days or you lost the balance outstanding.
If you hunt around you will find some variations on the theme, for example T-Mobile have a very special offer of 6 months for £20 if you buy from a source who knows about it. This seems to be unlimited (but people say 2 GB/month is safe) and works out quite well.
However PAYG Mobile Broadband is sold differently to voice services and time limited seems the norm.