Interestingly, it worked great on IE and Safari, but failed on Chrome and Firefox. What was even stranger was that everything worked properly when debugging in Visual Studio, but the failure occurred when the site was deployed to IIS.
So the failure occurred only when running Firefox or Chrome against IIS. The same website with IE and Safari worked great, and the same website on Cassini worked great in any web browser.
I ended up using WinDiff to compare the HTML output differences between what was served up from Cassini (Visual Studio during debugging) and IIS. There was one line that was different:
<form name="aspnetForm" method="post" action="/default.aspx?" id="aspnetForm">
<form name="aspnetForm" method="post" action="/" id="aspnetForm">