There was this Masi Clone Project: http://www.speedplay.com/speedplaylabs/masi/index.html

I've considered this as well but there is no way there would be enough of a market for any one bike to justify tooling up an assembly line for it. That leaves you with custom made. That's cool but not cheap. A local builder here in town who does really nice work would charge around $1900 to build a custom lugged, silver-brazed, steel frame and fork. That would include paint but not chrome which you would almost certainly need to reproduce an old bike so add another $500-1000 or so if you want that look. And that assumes that the correct lugs, fork crown, drop-outs, and bottom bracket could be found. If you need to custom make any of that the price would go up considerably I'm sure.

Now if you mean just building a modern steel frame to the same specs/geometry as some old classic frame, you could no doubt do that and sell a few but how would that be different than any of the current custom frame builders out there building really nice steel frames?

One thought I had if I ever won the lottery or had some rich uncle I don't know about die and leave me lot's of money would be to buy an actual old classic frame and then have a frame builder take it apart and rebuild it using the original lugs et al with the frame custom sized for me

One other possibility would be for an existing manufacturer to tool up to do a "retro" bike like the Ford did with the Mustang. Schwinn is apparently getting ready to do that with the Paramount, having a steel frame version built in Waterford and I have hope for that one while Bianchi is trying to pass off a cheesy Taiwanese bike of questionable quality with classic Bianchi styling queues that I think will be a disapointment.