This is one of the confusing question asked during interviews. Many people believe that values in textbox get cleared after postback when EnableViewState property of page is set to false. There are some controls like Textbox (which inherit interface called IPostBackDataHandler) has the capability to restore their values from the posted back.
If we take a look at the sequence in which asp.net life cycle events executes we will find that, there is an event called LoadPostBackData which get raised between LoadViewState and Load event. This one is responsible for retaining values in the controls between postbacks.
PreInit - >Init ->LoadViewState ->LoadPostBackData -> Load ->ControlEvents ->PreRender ->SaveViewState - > Render -> Unload
So the conclusion is textbox values (in fact values of all control which inherit IPostBackDataHandler) are not dependent on page viewstate and thus values will be retained even after postback regardless of what value EnableViewState property contains.