This machine has a completely typical file system layout and is using the directory per db option.

Tenseiga:~ avery$ mongo master.bb.mongo.ny
MongoDB shell version: 1.2.0
url: master.bb.mongo.ny
connecting to: master.bb.mongo.ny/test
type "help" for help
> db.printReplicationInfo()
configured oplog size:   7949.223731200002MB
log length start to end: 288058secs (80.02hrs)
oplog first event time:  Sun Jun 20 2010 06:24:29 GMT-0400 (EDT)
oplog last event time:   Wed Jun 23 2010 14:25:27 GMT-0400 (EDT)
now:                     Wed Jun 23 2010 14:25:29 GMT-0400 (EDT)
> use local                
switched to db local
> show tables              
pair.sync
sources
system.indexes
> 

This machine is using the per-db option, but the entry for 'local' in the data directory is a symlink to a directory on another partition:
Tenseiga:shopwiki.war avery$ mongo master.ui.mongo.ny
MongoDB shell version: 1.2.0
url: master.ui.mongo.ny
connecting to: master.ui.mongo.ny/test
type "help" for help
> db.printReplicationInfo()
configured oplog size:   67108.864MB
log length start to end: 392921secs (109.14hrs)
oplog first event time:  Sat Jun 19 2010 01:16:19 GMT-0400 (EDT)
oplog last event time:   Wed Jun 23 2010 14:25:00 GMT-0400 (EDT)
now:                     Wed Jun 23 2010 14:25:00 GMT-0400 (EDT)
> use local                
switched to db local
> show tables              
> 

Using db version v1.4.2, pdfile version 4.5