google.txt: I don’t know how well this is kept up-to-date, but it purports to be a list of all the IPs the Googlebot comes from.
This is of interest after the last post, because Google says you can detect the Googlebot User Agent and bypass login pages to index content behind subscription walls. But it also means that anyone can bypass your login page as well by changing their User Agent.
But, if this is an accurate list of Googlebot IPs, then you could detect both the User Agent and the IP. The trick, of course, is to make sure you have a an up-to-date list of IPs. I imagine it changes a lot, but of the 281 lines in this file, several are Class C subnets, which would encompass ~250 other IPs, so the actual number of potential IPs in this list is into the thousands.