Running fsync+lock on config server during backup results in blocked inserts

The documentation recommends doing an fsync+lock of a config server when taking a backup of a sharded cluster, but all of my tests using this method seem to result in blocked inserts after a few seconds ( I think when it needs to split the chunk ).

http://www.mongodb.org/display/DOCS/Backing+Up+Sharded+Cluster

Environment
Ubuntu 10.10
MongoDB 1.8.2
3 shards
3 replica set members per shard
3 config servers
1 mongos
chunksize=1 to force splits earlier

Steps to reproduce
1. Stop balancer
2. Verify balancer stopped
3. Run test script to generate inserts
4. fsync+lock config server
5. Wait a few seconds for block

I think I can work around the issue by shutting down the config database instead of doing an fsync and lock, but wanted to check to see whether or not this was a bug or just something missing in the documentation.

Thanks