Bogus!  OK, ASP.NET throws an exception if you try to reference the Page.IsValid property before either calling Page.Validate() or posting back based on a control that has a “CausesValidation” property of true.  The problem is that you can set AutoPostBack to True on drop-down lists, but it DOESN'T HAVE A CAUSESVALIDATION PROPERTY!  Why not??  So, if you are letting your drop-down listboxes cause a postback, be sure to call Page.Validate() before checking the IsValid property.