I got this from 20 different IPs today:
216.245.195.90 - - [08/Jan/2009:06:45:47 -0600] "GET /nonexistenshit HTTP/1.1" 404 7571 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.5) Gecko/2008120122 Firefox/3.0.5"
216.245.195.90 - - [08/Jan/2009:06:45:47 -0600] "GET /mail/bin/msgimport HTTP/1.1" 404 7225 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.5) Gecko/2008120122 Firefox/3.0.5"
216.245.195.90 - - [08/Jan/2009:06:45:47 -0600] "GET /bin/msgimport HTTP/1.1" 404 7227 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.5) Gecko/2008120122 Firefox/3.0.5"
216.245.195.90 - - [08/Jan/2009:06:45:47 -0600] "GET /rc/bin/msgimport HTTP/1.1" 404 6979 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.5) Gecko/2008120122 Firefox/3.0.5"
216.245.195.90 - - [08/Jan/2009:06:45:48 -0600] "GET /roundcube/bin/msgimport HTTP/1.1" 404 7174 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.5) Gecko/2008120122 Firefox/3.0.5"
I have RoundCube disabled in WHM for all accounts, but if you use it you might want to check RoundCube here. or this thread at WebhostingTalk.