[Update: This blog post sets up the problem, though it appears I was pointing the finger the wrong way. It turns out the problem was caused by Google Toolbar. See my followup post for more details.]
The JavaScript / JScript onunload event is notoriously unreliable from browser to browser. Reading though various USENET posts leads me to believe that the behaviour is slightly different between Mozilla derivatives, Opera, and IE. That sucks but is nothing new. What is, however, annoying is that behaviour differs on the same browser.
A few months ago I coded up some functionality that persisted state back to...