PubSub LinkRanks seem to be very sensitive to very recent activity which means one's rank can jump around a lot. I'm guessing this is particularly true at the long tail where just one link can leap frog you over hundreds of thousands of fellow bloggers.
Yesterday I was 938,610, today I am 89,060. I've been sub-100,000 before but I also spend time around the 1,000,000 mark if I haven't been linked to in the last week or so.
Oddly, Trevor Cook and others are reporting their rank has dropped recently. Perhaps some highly weighted bloggers just dropped out of the time-weighted window of referrers for their sites.
Or maybe PubSub have changed their algorithm. They say they are still refining it.
Incidentally, I'll use the recommended PSI so PubSub know I'm talking about them.