There are some hilarious, long threads on this topic on bikeforums. It's highly variable depending on terrain and bike, and people almost always overestimate their averages (i.e., look down often during a ride, I'm doing 21 mph, thus I "average" 21 when, including stops and slow periods, it might actually be 17)..
Here's my log #s for this year so far:
2013 totals
Bike: 128h 55m 21s - 2509.76 Mi
So, about 19.5 mph for a long term average for the year so far. To give that some context, I live in flat Florida, I'm a master's racer who does a lot of fast group rides, and then I also commute on a loaded touring bike at much slower speeds.