Either way, it would be pointless splitting them up now...
Google is a registrar and with the AdSense ID's as well, they know exactly who the sites belong to.
Some people seem to come unstuck with this, and others get away with it; I guess Google are seeing your sites as reasonable quality if they've been there a while and are paying out.
Google has no intrinsic problem with you owning 1,000 sites; as long as they add to their searchers experience (in Google's eyes) - and therein lies the crunch. Google can be entirely subjective in their idea of what constitutes a qulaity site.
I owuld personally want to spread the sites over multiple AdSense accounts (different companies) and different registrars/hosts/servers as you grow larger. It's hard to know when you're pushing your luck!
The key reason for using multiple IP's for hosting multiple sites is when you want to use the as link-network sites; in that scenario you need a lot of IP's.