There's at least one widget out there (ChumbyChat) that has a soft keyboard written in Flash.
The big issue is that chumby does not come with a stylus and widgets are generally designed not to need one. In the cases where some input is needed, for instance with Wifi configuration, it puts up special-purpose "keyboards" to allow input of the appropriate characters.
As an example, if you have to enter an IP address, it puts up a keyboard that only has the digits 0-9, the '.' and backspace, and the data is validated as you type it - that makes it impossible to enter an invalid address.
Most of the configuration of a chumby's widgets happens on the website, and the chumby acts as a player for those configurations - in general, input that's complicated enough to require a keyboard is discouraged.