Since Grimlock for BuddyPress v 1.3.5, you can now create a new xProfile field and sync it to the WP author bio. This feature only work with text areas.

First, go to your WP admin and navigate to Dashboard > Users > Profile Fields. then, click on Add new fields or Edit Group.

To sync both fields, simply check the required box. (Please see attached.)

All done. Yes, it is that simple.