Well, I think I’ve found the bug that makes IE not work so well with Booko.  The section of the page which displays the prices is updated via AJAX (with a GET post), but the first chunk of text for that section (with the spinning icons) gets cached by IE. The solution is to add an Expires header to the response. The actions which respond to GET requests got the following code:

headers["Expires"] = "Mon, 26 Jul 1997 05:00:00 GMT"

I should probably put a more correct value in there – but it’s always going to be set in the past at some point.