One way to compare road bikes and roller blades is as wheeled vehicles with the same engine but different transmissions. The main advantage of a road bike is that it has substantially more efficient gearing than a guy on roller blades. A way to put the point is that a roller blader is, transmission-wise, the rough equivalent of a cyclist on a fixed gear machine with a low gear, or, at least, a lower gear than is used by faster road bikers pushing along the flats.
Still, roller bladers are capable of some impressive speeds on the pavement, definitely.
A quick look at the world records for inline speed skating on the road here:
http://en.wikipedia.org/wiki/Inline_speed_skating#Road
The world record for 1k on the road is 77 seconds, which is 29 mph. The world record for 10k comes out to 27 mph.
These records are a little bit slower than the equivalent records for ice skating, which are 1k at ~33 mph and 10k at ~29 mph, but not by as much as I would have guessed.
http://en.wikipedia.org/wiki/List_of...ng_records#Men
Good roller blades must be pretty efficient to come this close on pavement to blades on ice.
These are the world records, of course, so even a well-trained roller blader you might run into on an urban path will be significantly slower than this. Still, a well-trained blader on good blades can be expected to keep up impressive speeds on pavement for quite a while, Abhibeckert's 40kph being a striking case in point. So, yeah, some bladers will be faster than recreational cyclists or commuters cruising along.