According to Mikael Colville-Andersen, of Copenhagenize Consulting, approximately 80% of people in Copenhagen store their bikes outdoors 24-7-365. This despite the fact that bike theft is rampant. When asked for advice on combatting rust, he replied, "the more rust the better."
One thing interesting to note is that the Danes generally ride extremely heavy bicycles. On the surface this would seem like a barrier to making cycling convenient, but it actually is one of many reasons that so many people ride in cities like Copenhagen. If bikes there were lightweight, more people would carry them indoors, in turn making utility cycling inconvenient. Just as there's "safety in numbers" in terms of riders, a similar phenomenon occurs regarding theft. If there are 500 bicycles locked up overnight within a one block radius, what's the chance that YOUR bike will get stolen? Again, theft is commonplace in Copenhagen, but the Danes accept it rather than fear it.
Obviously, any of you out there that spent $1,000+ on a luxurious, unnecessarily lightweight racing bike would never lock your bike up outdoors overnight. This post is geared more towards those with a utility bicycle used for quick errands to destinations generally 0.5 to 3 miles away. Having a bike locked up in front of your doorstep is a HUGE convenience for anyone who'd otherwise have to carry the bike up steps.
This is totally hypothetical, but I really do think that if bikes weighed as much in the US as they do in Denmark, cycling numbers would skyrocket in US cities. Basements and garages thwart all attempts to lure in would-be riders.