I actually noticed this around christmas time. Every now and again I head over to uceprotect.net to see how many spammers are on the same IP Rangers, as well as same ISP..
Over xmas, for every IP I tested (work, home, etc) I saw the graphs nose diving.
Check it now:
http://www.uceprotect.net/en/rblcheck.php?ipr=84.92....
Its a big slope up. Really interesting