BuddyPress.org

Figured this out, finally. Turns out that the file and function to modify was:
wp-contentpluginsbuddypressbp-corebp-core-filters.php
–>function bp_core_activation_signup_user_notification( $user, $user_email, $key, $meta )

Isn’t it redundant to have two functions so similar to each other? Both the above function and
wp-contentpluginsbuddypressbp-corebp-core-signup.php
–>function bp_core_signup_send_validation_email( $user_id, $user_email, $key )
are extraordinarily similar to each other. It’s kinda frustrating.

I’d consider developing a plugin, but perhaps I’ll wait for the first release of WP 3.0.