Hi, I’ve just released a new project called PhySwitch. It’s a simple app switcher using volume buttons to change between applications. Check the video demonstration here.
You can configure the action to activate PhySwitch using Activator (in Preferences.app). PhySwitch works with iPhone 3G/3GS/4 and iPod touch 3g/4g. Requires firmware 4.0 or higher. Available in Cydia Store for US$1.99.