Is it true that the next caldera-forming eruption of Yellowstone is overdue?
No. First of all, one cannot present recurrence intervals based on only two values. It would be statistically meaningless. But for those who insist… let's do the arithmetic. The three eruptions occurred 2.1 million, 1.3 million and 0.64 million years ago. The two intervals are thus 0.8 and 0.66 million years, averaging to a 0.73 million-year interval. Again, the last eruption was 0.64 million years ago, implying that we are still about 90,000 years away from the time when we might consider calling Yellowstone overdue for another caldera-forming eruption.

The least likely but worst-case volcanic eruption at Yellowstone would be another explosive caldera-forming eruption such as those that occurred 2.1 million, 1.3 million, and 640,000 years ago. However, the probability of such an eruption in any given century or millennium is exceedingly low.
http://volcanoes.usgs.gov/yvo/about/faq/faqactivity.php#notoverdue