If there was ever a single piece of software I hated with a passion, it would be Internet Explorer. I had let my seething resentment for this abomination settle for a while, as I do all my browing and development using FireFox.
Doesn't matter; this page uses a DatePicker component, nested within a few tables for layout (a new reason why tables for layout is a bad approach) and apparently that's enough to trigger this bug in IE.
Side note: I talk a lot about the importance of Feedback, that tools should clearly identify problems and guide you to solutions. On a grading scale of A - F, IE would receive the grade take out back and put down like a rabid dog on this issue. And many others.
I have a couple of leads on this bug:
I'm going to see if using the Tacos DatePicker will work better than the built-in Tapestry DatePicker.
Update: The Tacos DatePicker did the job. Better yet, the latest DatePicker (from Tacos 4 beta 2) is simply stunning from both a visual and a useability perspective.