Dolby says, and this is a quote from:
http://www.dolby.com/ht/Guide.HomeTheater.0110.html#s3.2
The surround speakers should be placed alongside and slightly to the rear of (but not behind) the prime seating position; well above ear level, to help minimize localization effects; and aimed directly across the listening area, not down at the listeners. This arrangement creates a diffuse, enveloping surround soundfield throughout the listening area, like that in cinemas and in the dubbing theaters where soundtracks are mixed. If the speakers are too far forward, you won't get sufficient rearward effect, and if the speakers are too far back, there will be a loss of envelopment and integration of the surround information with the whole soundfield.
Dolby.com has a lot of various good info and is worth a look.
