I have noticed this on and off for some time now. I have not been overly concerned because it hit random domains on a sporadic basis.
I run a robot alert cgi on my logs every half hour that looks for certain user agents as well as any hits to the robots.txt file. Then the script compares the ip against a know robot ip list. It will email me if anything is amiss. Today, 188.8.131.52 hit every domain I own (lots). Now I am concerned.
I can't find anything about this ip other than it is owned by Cisco Systems.
Have the same type of thing set up on our systems. Works great for catching people trying to steal your pages or new search engine robots!
Since a large part of the net is running off Cisco equipment, I imagine they are just running server/connection integrity checks. If they are just doing small checks I'll leave them be, but if they get out of control they're going to get tossed in the banned user agent pile.