Since journaling is now enabled by default (1.9.2+) then a repair will fail if the lock file has data from a previous run without journaling on. A repair should run even if there are no journal files when journaling is on (now default).

FAILS:
$mongod --port 7777 --dbpath /tmp/repairme/ --repair
Wed Aug 17 16:14:08 [initandlisten] MongoDB starting : pid=17963 port=7777 dbpath=/tmp/repairme/ 64-bit host=lamest.local
Wed Aug 17 16:14:08 [initandlisten]
Wed Aug 17 16:14:08 [initandlisten] ** NOTE: This is a development version (1.9.2) of MongoDB.
Wed Aug 17 16:14:08 [initandlisten] ** Not recommended for production.
Wed Aug 17 16:14:08 [initandlisten]
Wed Aug 17 16:14:08 [initandlisten] db version v1.9.2, pdfile version 4.5
Wed Aug 17 16:14:08 [initandlisten] git version: be8931b7eee5f6efa111cd0ad8f45ccfcffca6ca
Wed Aug 17 16:14:08 [initandlisten] build info: Darwin erh2.10gen.cc 9.6.0 Darwin Kernel Version 9.6.0: Mon Nov 24 17:37:00 PST 2008; root:xnu-1228.9.59~1/RELEASE_I386 i386 BOOST_LIB_VERSION=1_40
Wed Aug 17 16:14:08 [initandlisten] options:

{ dbpath: "/tmp/repairme/", port: 7777, repair: true }

**************
old lock file: /tmp/repairme/mongod.lock. probably means unclean shutdown,
but there are no journal files to recover.
this is likely human error or filesystem corruption.
found 1 dbs.
see: http://dochub.mongodb.org/core/repair for more information
*************
Wed Aug 17 16:14:08 [initandlisten] exception in initAndListen: 12596 old lock file, terminating
Wed Aug 17 16:14:08 dbexit:
Wed Aug 17 16:14:08 [initandlisten] shutdown: going to close listening sockets...
Wed Aug 17 16:14:08 [initandlisten] shutdown: going to flush diaglog...
Wed Aug 17 16:14:08 [initandlisten] shutdown: going to close sockets...
Wed Aug 17 16:14:08 [initandlisten] shutdown: waiting for fs preallocator...
Wed Aug 17 16:14:08 [initandlisten] shutdown: lock for final commit...
Wed Aug 17 16:14:08 [initandlisten] shutdown: final commit...
Wed Aug 17 16:14:08 [initandlisten] shutdown: closing all files...
Wed Aug 17 16:14:08 [initandlisten] closeAllFiles() finished
Wed Aug 17 16:14:08 dbexit: really exiting now

SUCCEEDS:
$mongod --port 7777 --dbpath /tmp/repairme/ --repair --nojournal
....