It is the most normal thing in nomination races to announce when you've clinched the nomination. McCain did it earlier this year. Kerry did it in 2004. Gore and Bush did it in 2000. ETC.
Correct me if I'm wrong, but I believe McCain celebrated once he actually reached the requisite number of delegates, not a percentage of that number. It just reeks of desperation, imo.
You would know...
Including a certain Bill Clinton in 1992.