Ever since Google became a registrar (admittedly not for the purposes of registering domains) last February, folks have been speculating about what they can do with that data. The typical buzz is that Google wants to use it to ID link networks, domains that are related, etc, but there can be more to it. Nick Wilsdon (writing at Jim's blog) gives some details:

It is also important note that several pieces of information are not included in domain privacy services. These are the current Registrar, record creation date, expiry date, renewal date and the name servers associated with the domain. A system could be set to look for one or more changes in this data as an indication the domain may have changed hands. Changing too much of this data plus a domain privacy service appearing on a domain might raise a flag or reduce Google’s trust in the domain.

They would especially be watching domain which are about to drop (expire) as this would be the likely time they would change hand. The system would be looking for status changes on WHOIS records, such as DETAGGED, REDEMPTION PERIOD, or PENDING DELETE. These would all indicate the registrant had failed to renew their domain. None would be hidden by domain privacy contracts.

His post is worth a look, as I believe, as do Jim and most of the community, that Google uses this data for precisely these (nefarious) purposes.