TL;DR - Sitecore 6.4 (and possibly other versions using SQL-based analytics) do not want you to truncate the IPOwner table. Analytics stops working.
Lo and behold, we discover today that one of our Sitecore applications hasn't been collecting analytics for a while. Investigation of the database revealed - gasp - no data in the IPOwner table !!!
The Sitecore logs revealed a multitude of Foreign Key constraint violations; specifically the foreign key IP.IPOwnerID - IPOwner.IPOwnerID. It's trying to add a row to the IP table using an IPOwnerID that does not exist.
Wait, what ? There aren't any rows in IPOwner ?
Suspecting magic GUID foul play and thinking that Sitecore is perhaps expecting a row to be there, we crack open a fresh copy of Sitecore 6.4's OMS database and sure enough, hiding in there was this little fella: -
And it all makes sense. While waiting for MaxMind GeoIP service to return something, Sitecore is using this guy as the IPOwner for a new session until the queued service lookup completes (hopefully), so it can write some meaningful location data to the IP record.
Added this row.. tested the site.. analytics starts recording stuff !
So now we know. Delete all the IPOwner rows if you want, just make sure this row finds its way back in.