http://docs.mongodb.org/manual/tutorial/recover-data-following-unexpected-shutdown/ states:

If the mongod.lock file in the data directory specified by dbpath, /data/db by default, is not a zero-byte file, then mongod will refuse to start, and you will find a message that contains the following line in your MongoDB log our output:

I tested this, and mongod started up just fine when I had a pre-existing lock file. The server code appears to only assert when the file cannot be opened or a lock obtained by the current mongod process. In my case above, I didn't have another process (mongod or otherwise) running with a lock on that file, so the new mongod happily started and took over the old lock file.