Frank, that's an interesting observation. I have seen this too so I just checked about 10 Columbus and 531-built bare frames and found only 2 that have seamed headtubes; a Durifort-built Bertin and a 531 (3-tubi?) Motob Grand Record. I suspect that large manufacturers saved a little dough without much of a weight penalty or possible repercussions. That said, I doubt a top-of-line frame would have a seamed headtube and I'd be a little suspicious if I were being asked to pay top dollar.
I'm a little suspicious anyway ever since I bought a "vintage" Eddy Merckx in the Bay Area for 80 bucks a couple of years ago when I was still green. After getting it home, I could find no provinance, frame weighed and "rang" like a lead pipe. I later learned that these counterfeits were/are common in Italy for the unsuspecting. Nice-lookin' frame though.
Jack Romans
Sacramento, Calif