From WBSPenguins.com:
Eaton, 35, spent the last two seasons with the New York Islanders, where he posted one goal, six assists and 18 penalty minutes in 96 games.
The fifteen-year pro skated four seasons with the Pittsburgh Penguins from 2006-2010 after being signed as a free agent on July, 3, 2006. Eaton compiled 31 points (7G-24A) over 218 games for Pittsburgh. He notched four goals and three assists in the 24 playoffs contests as the Penguins claimed the 2009 Stanley Cup.
Look at the end of the lockout from the Baby Pens perspective- Robert Bortuzzo, Brian Strait and Simon Despres all left for Pittsburgh and one way or another did not return. That's a huge chunk of your defense to just up and disappear, possibly for good.
For the Pens, it might just be filling that minor league depth and giving an old friend a chance to show his skills. Eaton had a rough year with the Islanders last year, but given the lack of defensive strength on that team, almost anyone probably would have had a rough year.
Eaton gets to keep playing, and maybe if there's more injuries get an NHL team to look at giving him a full contract. For Pittsburgh it might not mean much at this point, but it's definitely something to keep half an eye on.