Having the same problem. Well, on the iPhone, the home button property is stored in a file called com.apple.springboard.plist, but I was looking through an iPod touch version of the file this morning, and it's nowhere to be found in there. I thought I might be able to take the doubletap URL from the touch and put it on the phone, but I couldn't even find the touch URL. I hope someone else figures it out...
I'll probably just get Bluetooth headphones w/ media controls on them to solve this problem.