You can setup your BWC to leave the message on the server thata way after its been pushed to your BB you can still receive a copy on your Mac, you can also setup your mail client to do the same. If you own your own domain some hosts mine for example GoDaddy allows you to bcc: a copy of all message received, I find this option the best as I receive a copy in Entourage and a copy is sent to
skipping the BWC altogether allowing for instant reception of messages.