I’m writing a blog on this topic, (inspired by this forum post! 
that will publish at 10AM tomorrow. Basically, you can send sitewide notices, which are built into BuddyPress and sent by the admin user from the Messages > Compose screen. Or you can sent a PM to all users by using this plugin:
http://wpmu.org/wp-content/uploads/2010/07/bpmessage.zip
Install this plugin and you will find a new dashboard menu item under Settings >> Mass Messaging. This will allow you to send a Private Message to all of your users at one time. I found this little gem in a buried thread in the BuddyPress forums. It’s not available in the WordPress repository. The only change I’ve made was to the post URL, as it didn’t work for me.