If they were registered, it would go a ways towards putting bike thieves out of work. For instance, if whoever stole my bike last week was stopped and required to show registration, it would become clear that the bike is actually MINE and the fucker would be thrown to the dogs like s/he deserves. (Not that I’m bitter. :()
But so would the opposite strategy of a large fleet of communally-owned bikes.
Overall I agree that bike registration/licensing would definitely decrease ridership. I don’t drive because I don’t have a license. I can’t see how that would be different with bikes. Part of the joy of cycling is that it’s so accessible: anyone can do it, even if you won’t/can’t buy one, there’s usually someone you know with a spare one you can borrow. It would definitely discourage casual cyclists who wouldn’t want to go through whatever process is required to get registered, just to go for a spin along the lakeshore on a sunny day.
As a regular bike commuter I am of two minds on the subject of whether it would be a good thing or not.
One mind says that people should be encouraged to cycle by all means necessary: get cars off the road, get people fit, etc etc.
The other mind regularly gets pissed off at the Sunday-driver-cyclists who ride dangerously (particularly on Harbord St. during rush hour, why do they congregate there?), thereby (a) putting me and everyone else in danger, and (b) giving me and other safe cyclists a bad rep. If you needed a license to bike, this problem would be solved.