What are your platform and version ?
With the 4.2EA beta there were known issues of connection stability with the bundled JDBC version.
When was the last time that this happen? It's possible that you've exceeded the set active connections or your network might be faulty.
SQL Developer is running on windows 7 64bits and version 3.1. This error only occur to a one user. He try in differents stations using your user and nothing.
Today was the last time happen. He have one active connection and multiple windows with .sql open.
This problem sometimes occurs, when he's working excuting any script.