Yes, because when the film was initially released they forgot to put the copyright information & logo on the print (which used to be one of the stipulations), so it was never under copyright to begin with and Romero has never made a dime from that film. That's the main reason he did the remake in color back in the 80's, so he could get some money from it. That's also why you can find about a million different VHS & DVD versions of the film...you could dub a copy and sell it yourself if you wanted.
Found this on IMDB:
At the time of the film's release, any work that did not include a copyright notice was assumed to be public domain. Since the film makers forgot to include this notice, the film slipped into the public domain. In was not until 1 March 1989 that a copyright notice was no longer required.