I have read reference to a ferry from Nafplion to Athens. Has anyone taken it, or have any information on it? I assume it only runs in the summer? Is it direct, or do you stop at another island on the way? We are taking the bus to Nafplio, and I thought it would be nice to mix it up by taking a different way back.

There are no ferries from Nafplio. Excursion vessels may run in summer to Spetses, Idra or Poros (www.pegasus-cruises.gr), and you can then get a ferry on to Pireas.
You can also take a bus from Nafplio via Kranidi to Porto Heli and Kosta. Hellenic Seaways (www.hellenicseaways.gr) run fast craft from Porto Heli to Pireas, or from Kosta, it's a short ferry crossing to Spetses which has both ferries and fast craft to Pireas.
The journey would certainly be worth considering if you wanted to visit one of the islands en route.

Thanks! We very well may consider Spetses en route. We have the whole day to get back to Athens, so it would be a fun way to go.

http://www.greecetravel.com/nafplio/ refers to a quaint old train that takes 3 hours to Athens - is the bus much faster than that? Also at bottom, they refer to to summer flying dolphin ferry connections.

There are two train connections a day from Nafplio to Athens (www.ose.gr) and the fare is a bargain 4.80 euros.
Hellenic Seaways' predecessors used to run hydrofoils from Nafplio but I don't think they run any more - Nafplion certainly doesn't appear on their web site.
There are a variety of options for the journey, involving buses, boats and/or trains. The choice might depend on the time of year, the weather, etc.

Does the train follow the same route as the highway (and bus?) or would we get some different scenery? The train sounds like a fun way to go. We are going to be there mid-May, so I doubt any real seasonal services would be running at that time. I will alrm myself with all the options, then we can decide the day before based on the weather. Thanks all!