TextBox ReadOnly in ASP.NET v2.0
If TextBox's ReadOnly property is "true", postback data won't be loaded e.g it essentially means TextBox being readonly from server-side standpoint (client-side changes will be ignored). If you want TB to be readonly in the "old manner" use TextBox1.Attributes.Add("readonly","readonly") as that won't affect server-side functionality.
This is documented here:
http://msdn2.microsoft.com/en-us/library/system.web.ui.webcontrols.textbox.readonly.aspx
And someone pointed that out as bug a while ago:
http://lab.msdn.microsoft.com/productfeedback/viewfeedback.aspx?feedbackid=60bb1325-9dc3-48e9-a32f-e211ea345862
Even though it's not even about EnableViewState but only about server-side ReadOnly property affecting postback data loading...