The link between Athens airport and the city centre is to close from February 14th for 'about 6 months', according to an official announcement on Wednesday. This is to allow work on new stations.
Buses to Syntagma and Piraeus will still be running and for most people this is probably the best bet.
This closure was anticipated several times last year but my guess is that this time it really is going to happen. If anyone is in Athens in the next few weeks I'd be interested to know how it is working.
I'll be there myself in mid March and will report back.