I modified the example combo.js in the form example directory.
It is possible to select an item even if it is readOnly.
just press space into.

thanks

[/code]
// Simple ComboBox using the data store
var simpleCombo = new Ext.form.ComboBox({
fieldLabel: 'Select a single state',
renderTo: 'simpleCombo',
displayField: 'name',
width: 500,
labelWidth: 130,
readOnly: true,
store: store,
queryMode: 'local',
typeAhead: true
});
[/code]