mongod --dur still preallocates journal files even when --noprealloc is specified

Tony1:~# mongodb-linux-x86_64-1.8.0-rc2/bin/mongod --dbpath db --dur --replSet rs0 --noprealloc
note: noprealloc may hurt performance in many applications
Thu Mar 10 20:49:04 [initandlisten] MongoDB starting : pid=1743 port=27017 dbpath=db 64-bit
Thu Mar 10 20:49:04 [initandlisten] db version v1.8.0-rc2, pdfile version 4.5
Thu Mar 10 20:49:04 [initandlisten] git version: 460d32f86ba837a4f1dd41cfeb65eddee41df65f
Thu Mar 10 20:49:04 [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
Thu Mar 10 20:49:04 [initandlisten] journal dir=db/journal
Thu Mar 10 20:49:04 [initandlisten] recover : no journal files present, no recovery needed
Thu Mar 10 20:49:14 [initandlisten] preallocateIsFaster=true 95.44
Thu Mar 10 20:49:23 [initandlisten] preallocateIsFaster=true 96.86
Thu Mar 10 20:49:33 [initandlisten] preallocateIsFaster=true 104.7
Thu Mar 10 20:49:33 [initandlisten] preallocating a journal file db/journal/prealloc.0
Thu Mar 10 20:52:49 [initandlisten] preallocating a journal file db/journal/prealloc.1