Hi
Have you thought about using the logdata from caching proxy's to check against this data and the other SURBL lists? The caching proxy's logfiles contain a very good view on which domains are used while surfing. While not all domains that are used are legit, I would be surprised if active spam domains were frequently visited over a period in the past. Those frequently surfed to domains shouldn't be whitelisted by default, but it could be a trigger to have a closer look before placing them on a list.
A very nice thing about this data is that it has a long period of usefulness even data from a few months ago will be very usefull, if not more usefull than the data of today or from the past week.
A caching proxy could also make good use from the surbl lists. Not caching spammers domains or even adding a delay of several seconds (or even adding a warning page) before returning the page could be usefull. Especially in non ISP use (organisations, companies,etc...) where there are less problems with blocking content.
Alain