This will replace the appearance of all TriggerFieldCells and their subclasses.
To replace the appearance of a single instance of a ComboBoxCell, you will need to subclass ComboBoxCell and set the value of the appearance field inherited from TriggerFieldCell. This can probably be done easily enough using an anonymous inner class.
That being said, I've filed an API review ticket against the team to see whether ComboBoxCell should indeed have a constructor that accepts an appearance. This review will give us a chance to take a moment to ensure that the current design is what we intended. I'll post a reply here if we decide to make any changes.
This enhancement has been added in the Ext GWT 3.0 Release Candidate. Please upgrade your copy of Ext GWT and try your scenario again. While we're confident that we've addressed this issue, please reply if you notice any continued problems after upgrading. Again, thanks for taking the time to report this issue.