There is an update for .NET framework 4 which allows you to specify the type attribute
http://support.microsoft.com/kb/2468871.
See feature 3
way down the page
Feature 3
New syntax lets you define a
TextBox control that is HTML5
compatible. For example, the following
code defines a TextBox control that is
HTML5 compatible:<asp:TextBox runat="server" type="some-HTML5-type" />