Android: How to enable/disable option menu item on button click?

Anyway, the documentation covers all the things.

Changing menu items at runtime

Once the activity is created, the
onCreateOptionsMenu() method is called
only once, as described above. The
system keeps and re-uses the Menu you
define in this method until your
activity is destroyed. If you want to
change the Options Menu any time after
it’s first created, you must override
the onPrepareOptionsMenu() method.
This passes you the Menu object as it
currently exists. This is useful if
you’d like to remove, add, disable, or
enable menu items depending on the
current state of your application.

E.g.

@Override
public boolean onPrepareOptionsMenu (Menu menu) {
    if (isFinalized) {
        menu.getItem(1).setEnabled(false);
        // You can also use something like:
        // menu.findItem(R.id.example_foobar).setEnabled(false);
    }
    return true;
}

On Android 3.0 and higher, the options menu is considered to always be open when menu items are presented in the action bar. When an event occurs and you want to perform a menu update, you must call invalidateOptionsMenu() to request that the system call onPrepareOptionsMenu().