I have the same problem with BP 1.2. When activity streams are disabled, the individual group pages show an error:
Fatal error: Call to undefined function bp_activity_post_form_action() in /public_html/wp-content/plugins/buddypress/bp-themes/bp-default/activity/post-form.php on line 1
Does anyone have a fix for this? Or am I the only one having it with bp 1.2 and WP 2.9.2?