BuddyPress.org

I have a similar environment to Andij: BuddyPress on top of WordPress (3.0b2), no subblogs, upgrading from 1.2.3 to 1.2.4.
Still customizing the BP_AVATAR_URL was not enough for me. I also had to customize BP_AVATAR_UPLOAD_PATH.

In the end, I have created the bp-custom.php file and added the two following lines (I have installed WP / BP at the root of my server):

define( ‘BP_AVATAR_URL’, ‘http://’ . $_SERVER . ‘/wp-content/uploads’ );
define( ‘BP_AVATAR_UPLOAD_PATH’, $_SERVER . ‘/wp-content/uploads’);

I hope the need for those two lines will be removed in a next release of BP…

Hope it helps