This BuddyPress plugin by modemlooper adds a ‘tweet’ button to activities, forum topics and blog posts, allowing users to ‘tweet’ the item.
Requires WordPress Version: 2.9 or higher Compatible up to: 3.0.1 Requires BuddyPress 1.2 or higher.
wpmu.org offers a modded version and a video of how the plugin works at: http://wpmu.org/new-buddypress-tweet-button-plugin-download-here/
Download and info: http://wordpress.org/extend/plugins/bp-tweet-button/