From Wired:
The Stagefright bug has quickly frightened cell phone manufacturers into action. It’s been just over a week since researchers alerted the public to the serious flaw that has been called the worst Android “bug ever discovered,”, and the major Android manufacturers have already taken concrete steps to fix it.
Source: Big Android Makers Will Now Push Monthly Security Updates | WIRED
The linked article includes a link to an app that can detect Stagefright vulnerability,