If your goal is have a "readonly select" - that is show the user what the choices are without allowing it to change and have it sent with the POST variables, you want to use "selected" on the option with the current value, and "disabled" on all the other options. This will essentially show the select with all its options but only allow the current one to be selected.
I find this more helpful than simply having a disabled select.
As others have noted, you still need to insure that the server side doesn't accept changes to the field (as with any readonly field).