@bbg: Well, being new to GWT development, I haven't used much Cells yet. If you have a sample code to share (how to add it to a Sencha text field), I would be glad.
Beside, I stands by my point: it makes, if not really "hard", non obvious to have this very common functionality.
@mtraynham: many thanks, it works well!
Indeed, it would be nice to have that in the core. I had to make a TextInputCell and a PasswordinputCell overriding onEnterKeyDown as you shown, to create the handler with the login action, to attach the handler to the cells and to create the input fields with the cells as parameters. A bit convoluted... But it works!