Which is not fully compatable/working with vbulletin 3.7 that was released previous to pp 6.1, on April 29 2008. And also is not much of a "rollup of minor fixes" as stated if its introducing new bugs along with those "new features" mentioned in the pp release announcement.
One Example;
http://www.photopost.com/forum/photopost-pro-bug-reports/136499-another-6-1-upgrade-error-post1223972.html
Another is this very thread and the editor issues, to name only two.