hmmmm.
If I remember correctly, the estimate of "every 500 years" is invalid. Averaging the estimated periods between the events doesn't mean anything when there is such a huge disparity between the longest known period between events and the shortest.
http://earthquake.usgs.gov/recenteqsww/Quakes/quakes_all.html
I haven't watched this for awhile so don't know if the activity by Mt St Helen is up or just same as usual.