Mongodb can't start and I'm getting the old lock file error in the log although journaling is enabled
and the log says it cleaned the journal files when it receive the machine shut down signal.

journaling is enabled by adding the following to /etc/mongodb.conf:
journal = true

This is the log file:

Fri May 27 09:49:13 got kill or ctrl c or hup signal 15 (Terminated), will terminate after current cmd ends
Fri May 27 09:49:13 [interruptThread] now exiting
Fri May 27 09:49:13 dbexit:
Fri May 27 09:49:13 [interruptThread] shutdown: going to close listening sockets...
Fri May 27 09:49:13 [interruptThread] closing listening socket: 6
Fri May 27 09:49:13 [interruptThread] closing listening socket: 7
Fri May 27 09:49:13 [interruptThread] closing listening socket: 8
Fri May 27 09:49:13 [interruptThread] closing listening socket: 9
Fri May 27 09:49:13 [interruptThread] removing socket file: /tmp/mongodb-27017.sock
Fri May 27 09:49:13 [interruptThread] removing socket file: /tmp/mongodb-28017.sock
Fri May 27 09:49:13 [interruptThread] shutdown: going to flush diaglog...
Fri May 27 09:49:13 [interruptThread] shutdown: going to close sockets...
Fri May 27 09:49:14 [interruptThread] shutdown: waiting for fs preallocator...
Fri May 27 09:49:14 [interruptThread] shutdown: lock for final commit...
Fri May 27 09:49:14 [interruptThread] shutdown: final commit...
Fri May 27 09:49:14 [interruptThread] shutdown: closing all files...
Fri May 27 09:49:14 closeAllFiles() finished
Fri May 27 09:49:14 [interruptThread] shutdown: journalCleanup...
Fri May 27 09:49:14 [interruptThread] removeJournalFiles
Fri May 27 11:03:26 [initandlisten] MongoDB starting : pid=668 port=27017 dbpath=/var/lib/mongodb 64-bit
Fri May 27 11:03:26 [initandlisten] db version v1.8.1, pdfile version 4.5
Fri May 27 11:03:26 [initandlisten] git version: a429cd4f535b2499cc4130b06ff7c26f41c00f04
Fri May 27 11:03:26 [initandlisten] build sys info: Linux bs-linux64.10gen.cc 2.6.21.7-2.ec2.v1.2.fc8xen #1 SMP Fri Nov 20 17:48:28 EST 2009 x86_64 BOOST_LIB_VERSION=1_41
**************
old lock file: /var/lib/mongodb/mongod.lock. probably means unclean shutdown,
but there are no journal files to recover.
this is likely human error or filesystem corruption.
found 4 dbs.
see: http://dochub.mongodb.org/core/repair for more information
*************
Fri May 27 11:03:26 [initandlisten] exception in initAndListen std::exception: old lock file, terminating
Fri May 27 11:03:26 dbexit:
Fri May 27 11:03:26 [initandlisten] shutdown: going to close listening sockets...
Fri May 27 11:03:26 [initandlisten] shutdown: going to flush diaglog...
Fri May 27 11:03:26 [initandlisten] shutdown: going to close sockets...
Fri May 27 11:03:26 [initandlisten] shutdown: waiting for fs preallocator...
Fri May 27 11:03:26 [initandlisten] shutdown: lock for final commit...
Fri May 27 11:03:26 [initandlisten] shutdown: final commit...
Fri May 27 11:03:26 [initandlisten] shutdown: closing all files...
Fri May 27 11:03:26 closeAllFiles() finished
Fri May 27 11:03:26 [initandlisten] shutdown: journalCleanup...
Fri May 27 11:03:26 dbexit: really exiting now