Buddy of mine, an engineer and bike fanatic, once did some work for Serrotta. He came away with a prototype titalnium frame sample from a manufacturer based on his specs. No markings on it at all.

I'd be real wary of a completely unmarked frame. First, I'd think "Stolen!" look for any indication that a serial no. had been removed. And then, it would nag at me that this was indeed some kind of manufacturing sample... which could be good or bad. Good, because a manufacturer might be trying to impress a buyer with excellent quality in welds and alignment; bad, because the potential buyer might have made substantial and critical changes between a sample and production frame.