1.) The only work around I'm aware of is if your set-top box has multiple outputs connect one of them to a different input on the tv. Then just change to the alternate tv input while sound is coming from the GTV.
2.) Otherwise, just connect the set-top box to an alternate tv input, instead of routing through the GTV. You lose picture-in-picture through the GTV, but the TV may be able to provide it.
Google and/or Logitech (And Sony - for those folks) really do need to provide for this type of arrangement anyway. Particularly for people who do not use set-top boxes (i.e. as all tvs sold in the US are HD now, and not everyone uses cable or satelite.) Personally, I have a second GTV in the bedroom, but don't have a set-top box there, just basic cable and over-the-air. It would be nice if the GTV knew to just switch inputs when I selected watch live tv. It's not that difficult.
Um... sorry for the ranting. I'll stop now.
Anyway, the two options above may not be optimal, but they are functional.
Logitech Media Server integrated with Music IP Mixer on a fit-PC2 running Ubuntu serving:
2× Logitech Revue
3× Logitech Squeezebox Duet
1× Logitech Squeezebox Touch
1× Logitech Squeezebox Boom