Berndt at Dreamchip has been working on such a thing for years now, and he has a little box called xbounds that can do this over WiFi for Maemo and Android. Don't know when they're actually planning to release it, it has been "in production" for so long now... I saw a demo on the N900 back in Summer 2010 and it was really good, but I guess he didn't want to release it for the N900 since it is discontinued...