Pressing up from the Channel's root screen does/should exit by convention. Hitting home is enforced by the system.
Interestingly, however, the "up to exit" functionality seems to have disappeared from the latest Netflix channel. I regularly get caught out by that.
My Channels: http://roku.permanence.com - Twitter: @TheEndlessDev
Instant Watch Browser (NetflixIWB), Aquarium Screensaver (AQUARIUM), Clever Clocks Screensaver (CLEVERCLOCKS), iTunes Podcasts (ITPC), My Channels (MYCHANNELS)