When backing up a wallet, is it required to shut down the bitcoin client? I am building a website, and would like to backup my wallet.dat file after every transaction occurs, but I would not like to have to shut down bitcoind every time I make a backup.

The common wisdom seems to be that you should shut down your bitcoin client when making a backup, but that doesn't seem absolutely necessary as long as the wallet has been fully written to and flushed to disk.

Would locking the wallet be enough to ensure that the wallet has been written to before making a backup? Are there any other techniques that websites typically deploy for making "hot backups"?
The Bitcoin client's RPC interface provides a wallet backup command. It's "backupwallet &lt;filename>".