There is a bus at the port to take you into Athen.
I am going to Athens in Sept 08, and need to visit, the war cemetery, but also to see the 'Sights', and Iam using a company called www.greektaxi.gr, researched the subject,and the company theough various sites, and they appear to have a good name, the proof will be in the pudding I guess. What I did find out is to keep away from greecetaxi, they appear from what I read to be 'Rip of merchants'. Greek taxi has a website as shown, will meet you at the port, but will ask you to leave credit card details over a secure site, in case of a no turn up. charges 250 for a full day and as there are 4 of us I don't think that is too bad, cruising athens in a aircon merc. Have a look, and enjoy the trip.