It was actually quite a big effort and, according to Calvin and other team members, would break existing apps as file indexing would have to change. I don't remember all the details anymore.

>It's a pity - Calvin Hsia could probably do it in a day or a weekend. Really, if MS thought about it, it would be helping developers stay on Windows by improving a Windows tool and without that much effort really. I see that COM (and I think even ASP Classic!) is in 64-bit versions sot it's not like they didn't do it for other legacy technologies.