General rule - if it's a text area that is sometimes editable, make it a disabled input field. For example, you have to select a country before you can type in your zip code. Before the country is selected, make the input field disabled. This This gives the user a clue that "I can interact with this field, but not right now". You can use the disabled input fields in combination with mouse over tooltips that explains why it is disabled.
If it's never editable - make it plain text!
This follows the same convention as other web interactive objects, like buttons. If a button usually is enabled, but becomes disabled it tells the user "in this state of the environment, I can't press the button because the variables aren't set right" - for example, one of many required text fields aren't filled correctly.