Top speed downhill (coasting/gravity) for me so far: 74.6 kph (about 46 mph).
Top speed on flat terrain so far: 42 kph (26 mph)
The later speed is mostly because I run out of gearing - I need a bigger cog up front (only have a 42 tooth at the moment). Based on the feeling in my legs, I think I could break 50 kph with some higher gearing. When not racing or trying to give myself a heart-attack, I'm quite happy with the 25-30 kph (15-18 mph) cruising speed I get during my daily commute.