I wouldn't really call Joey a thin client. It is a fully featured computer and runs all the code locally, the only difference is the data stream is coming from the hopper vs a local drive. A PC is not really overkill with respect to hardware.
That being said, Dish could write a "Hopper Client" to run on PC/GTV/Boxee/etc. I think we will eventually see such a beast as an answer to Direc's RVU, but likely as part of the next generation hopper, nothing I would make any plans on.
Just something to dream about for the future.