From the Google Mobile Blog:

You asked for it, so here it is. We’re happy to announce that in the latest update to Google Sync for BlackBerry, we’ve added two-way contacts synchronization. This new functionality will enable you to sync your handheld’s built-in address book with your Gmail contacts. This all happens in the background and over the air, so your information is always up to date, no matter where you are or what you’re doing.

Once you’ve installed Sync, all your information will be safe in your Google account. If you ever lose your phone or buy a new one, getting your address book and calendar to your new device is as easy as installing Sync. Current and new Google Sync users can try Google Sync today by visiting m.google.com/sync from their BlackBerry browser.