You won't be able to do it, at least not easily. In a normal OS X installation, the wireless driver is provided through a .kext which can be unloaded and replaced with one that supports monitor mode. On the iPod Touch, everything is compiled into the kernel. Unless the builtin driver somehow has monitor mode support, which I doubt to be the case, you're going to be out of luck. If you can find some way to load a monitor capable driver simultaneously, Marvell actually provides an open source Linux driver to start with.
If your goal is Linux on an iPod Touch for Wifi hacking, you're better off buying a device that already has Linux on it and going from there. There has been no progress made yet of porting Linux to this device or the iPhone. This is also going to be difficult given the nature of the highly specialized multitouch interface. I agree this would be cool, which was why I looked into it initially, so best of luck to you.