Re the last poster, does that definitely work with the Linux client? It doesn't work with the Windows client.
If you just have the single line, it'll come back right away with an error ("Connecting to [username] has failed."). If you create a newline after it, it does the same. Perhaps there's some other nuance to this.
The manual seems to agree with you though. Could it be in reference to Linux only?
--auth-user-pass [up]
Authenticate with server using username/password. up is a file containing username/password on 2 lines. If the password line is missing, OpenVPN will prompt for one.