Locking access to the data store is one way to provide safe concurrent access, but it starts to fall over as soon as you have more than a few scripts accessing the same Storage object. We have no locks in CouchDB, accomplished by using multi version concurrency control, a storage technique popularized by the PostgreSQL database. The MVCC approach would seem to be a simpler and more humane solution to the problem of concurrent storage usage.
From Fixing HTML 5 Storage by J Chris Anderson