Ok now I am getting really wierded out. I thought hmm maybe they are ramping up mediabot as the primary crawler as was mentioned since we have adsense on the sites this would make sense.
I go and grep out all of the mediapartners-google lines in my log and see a few of the tens of thousands that are on obviously different ip addies.
I trace them back and they lead to Microsoft Corp?
Microsoft is spoofing Mediapartners-Google? The crazy drop in indexing started less then 1 hour later. I see how your friend came up with Bingbot-Googlebot going out on a date. This is really getting strange now.
These IP addies are showing in my logfiles labeled as Mediapartners-Google
Less then 1 hour later almost all regular GBOT activity dies.