Sometimes, upgrading Oracle SQL Developer will not import your existing connections. You can have in the end an empty connection list. In order to import it, you just need to copy the connections.xml file from your old location like this:
"c:\Documents and Settings\[user]\Application Data\SQL Developer\system1.5.0.53.38\
o.jdeveloper.db.connection.11.1.1.0.22.49.35\connections.xml"
o.jdeveloper.db.connection.11.1.1.0.22.49.35\connections.xml"
to your new location like this:
"c:\Documents and Settings\[user]\Application Data\SQL Developer\system1.5.1.54.40\
o.jdeveloper.db.connection.11.1.1.0.22.49.35\connections.xml"
o.jdeveloper.db.connection.11.1.1.0.22.49.35\connections.xml"
Another version would be, when upgraded version of SQL Developer is asking for the path to the connections file, just provide him the path to the old version.
Good luck!