Wirelessly posted (iPod touch: Mozilla/5.0 (iPod; U; CPU iPhone OS 3_1_3 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7E18 Safari/528.16)
If you jailbreak, you can use Activator to assign different button presses and screen gestures to launch certain apps.
Without jailbreaking a double press of the home button will do nothing except fast app switching and you can't use any other shortcuts.