The goal was to see if deleting items would get me below a threshold, or if the threshold was irreversible.
The weird thing is that once I got that message, previously-deleted objects began re-populating the calendar. Calendar zombies had risen from the grave!
Clearly there's a cache of the deleted items. I have no information on when, how often, or if it gets cleared in a single session.
There is also some interesting behavior with old items. To see this, load 10-15 years worth of calendar data and then travel back to some month in the year 2000. The following unobtrusive message will display while the data renders (and it seems to take a while):
Added on September 6, 2011:
A variation of the above: cannot load your data -- come back when it's more convenient for Google....