Atwill's been peddling this for a long time. Has he qualified in anything since first publishing? I'm behind on my pseudo-history stuff. But, anyway, this 'theory' has been debunked pretty thoroughly. It's a fun idea to turn the gospels into a first century Da Vinci Code, but it's not something which stands up to serious scrutiny.
Robert Price's debunking is fairly thorough: http://www.robertmprice.mindvendor.com/rev_atwill.htm