Judging by recent spamassassin-users mailing list messages, Verizon is hijacking DNS responses that seem to be invalid and replacing them with their own responses (in oder to drive traffic to their search sites). Naturally this breaks SURBL lookups. If you are using Verizon's nameservers and are getting false positives, you may want to check into this.

Here's one reference:

http://www.freedom-to-tinker.com/?p=1227

Jeff C.