A few days ago, Danny spotted Google Product Search Results in the regular Google SERPs. The problem with this is that Google has asked others not to do this in the past, but they apparently weren't practicing what they preached.
The reason for the mistake was that the robots.txt was not updated; it disallowed the old Froogle and not the new Google Product Search directory location. In the comments, Matt says that it was in fact an oversight, and the robots.txt has already been updated.
Discussion at Search Engine Roundtable Forums.