With the recently published jailbreak for iPod Touch, combined with the SDL port for iPhone there should be little in the way for running Rockbox on it as an application, pretty much in exctly the same way I mentioned how Rockbox could be made to run on mobile phones.
It seems a suitable place to start this venture is the iphone-dev project page.
While the iPhone and iPod Touch aren’t 100% identical internally, it seems they’re similar enough to make the differences possible to ignore. Also, the fact that what everyone does is build applications that run under the normal Apple-provided OS, there’s no need to know or learn how to poke on the actual hardware so subtle differences in audio chips etc is abstracted away by the operating system even for applications put on the unit this way.
Update nov 2008: With the recent developments on the linuxoniphone blog, it looks like an iPod Touch version of Rockbox is now a lot more likely to be possible. Still, nobody has yet volunteered to start this work and I won’t even say that it is likely that anyone will make an attempt.