This is not the case if you are using Yahoo's tool bar or Google's auto fill feature in its tool bar, but IE is more stupid.
I fixed this issue my explicitly settings the AutoCompleteType attribute in the Textbox control.
Example textbox for first name field.
<asp:TextBox ID="txtFirstName" CssClass="input" runat="server" AutoCompleteType="firstname"></asp:TextBox>
By setting the AutoCompleteType attribute the problem was corrected.