Just in case anyone's interested, I've just popped back from Germany with some cruise control data on mpg.
Using Torque (great app) I logged various sensors including speed and mpg (instant) and drove back and forth along the same stretch of road a few times, at different cruise control speeds. My data, summarised:
50mph - 42.1mpg
60mph - 38.6mpg
70mph - 32.0mpg
80mph - 28.1mpg
Obviously there's some margin of error here due to things like wind changeability, polling speed (which was every couple of seconds) et al, but gives a rough guide for those who care.
Tested with a 2002 Golf with the 2.8L 24v V6 4MOTION and 138k on the clock. And I changed the spark plugs this morning.