I covered a DigitalPoint thread which uncovered several domains that was able to rank billions of pages at the top of the Google results within a couple of weeks. The methods deployed to rank the pages seemed to include excessive use of subdomains, cloaking, content theft scraping, alexa traffic boosting and blog comment spam. I listed the documented steps here. Some suspect that Google's new URL handling with the big daddy update allowed "old school" cloaking to begin working again.
A Threadwatch post shows screen captures of the spam and also has a comment from Google representative, Adam Lasnik. Adam directly responds to over 5 billion pages of this domain being indexed, saying:
We have noticed that some site: queries are showing bizarre results and it's turned out to be tied to a bad data push. We're fixing it now.
Yes, we are aware of the site command issues (Google's mentioned them itself). That may mean it is far less than 5 billion pages indexed in this case -- but still, plenty of pages got through.
If the site command is the issue or even if it is not, this is still indicative of other substantial problems plaguing Google that are making the rounds on discussion board and blogs lately.