But as computing power increases, that minimum strength is being pushed out. bcrypt lets us hold the line by keeping pace with computing power— couldn't it also give us the ability to push back?

How many users are using the 100,000 most common passwords? 1,000,000? As it stands today, anyone using one of those is instantly compromised the moment the hash file is accessed. With a truly variable work factor, you could theoretically ensure the safety of any password, regardless of strength.

It obviously gets silly toward the far end (make "password" take three months to hash?) And maybe it gets silly a lot sooner than I'm thinking. But surely it could be pushed back a little, yes? Make one of those 1,000,000 passwords intractable, and you're protecting thousands of users from attack.

False.

Collision attacks don't apply to many situations but are much easier to execute, for example a MD5 pre-image attack requires approximately 2^128 steps but a collision attack requires only about 2^64 steps. This is why MD5 is totally unsuitable for collision resistance, and in fact has already been successfully exploited to fabricate a real-world CA certificate, but still puts up mild resistance to password cracking. Not that I'm recommending you use it or anything -- do what the nice gentleman says and just use bcrypt already!

Please disregard my comment above.

How? If hashing one password takes one second, and you have a dump of a thousand users, it will take you a million seconds to try just 1,000 common passwords on that list.

Are you thinking of unsalted hashes, perhaps?

So really I think you were demonstrating my point :)