SQLDeveloper 3.0 production is supplied with the thin driver 126.96.36.199
it needs 188.8.131.52 version to use thick which has a few additional features over thin.
To avoid confusion with other Oracle software you can set the ORACLE_HOME in the session you start sqldeveloper in as described below:
The version mismatch is often because SQLDeveloper is trying to use the 'thick' driver and 184.108.40.206 thick is not available,
1/(uncheck Tools->preferences->Datababae->advanced->use OCI/thick driver, and
2/If necessary, set ORACLE_HOME for your cmd.exe session where sqldev is started from to where sqldeveloper is where %ORACLE_HOME%\jdbc\lib exists, (so SQLDeveloper does not try your existing ORACLE_HOME) and
3/use Connection Type TNS -> Network Alias.
4/You may need to set Tools->preferences->Datababase->advanced->Tnsname directory to the directory containing tnsnames.ora
Edit 25 Sep 2011
P.S. PATH (and LD_LIBRARY_PATH ??) settings may be needed to find the new .dll (Windows) and .so (Linux) files.