I'm not going to tell you that I understand the answer... it's posted in full here:
http://stackoverflow.com/questions/7228344/postback-doesnt-work-with-aspx-page-as-default-document
but I can tell you what I did to fix it: I placed this code in the Global.asax
Sub Application_BeginRequest(sender As Object, e As EventArgs)
Dim app As HttpApplication = CType(sender, HttpApplication)
If app.Context.Request.Url.LocalPath.EndsWith("/") Then
app.Context.RewritePath(String.Concat(app.Context.Request.Url.LocalPath, "default.aspx"))
End If
End Sub