Just a thought: given the dynamic nature of the sc.surbl.org data, it might be useful to have a listing history for the SURBL+ checker (http://www.rulesemporium.com/cgi-bin/uribl.cgi)
Something along the lines of domain xxxx currently (listed|notl listed) in sc.surbl.org - added ddmmyyyy hhmm - removed ddmmyyyy hhmm - added ddmmyyyy hhmm - removed ddmmyyyy hhmm
This way it would be possible to check whether old domains are being left dormant until they work their way out of the list and then being resurrected later on.
Another thought was to make a separate historic list of domains that are removed from sc.surbl.org when there are no new reports. People who want to do more aggressive filtering could also use the historic list in addition to the other lists. The risk of FPs is still low because I can't forsee there being many "reformed" spammer domains.
John