In firmware 2.x Apple added a new protection mechanism to prevent unauthorised modification of the iTunesDB database file on these devices. This is done via a special signature in the file, it has been referred to as a “hash” but it is more complicated than that.
At this time the complete method to generate a valid signature is not publicly known. Apple have tried to make it as hard as possible for someone to reverse engineer the method. Without having a way to generate a valid signature, full support of these devices/firmware versions is not possible. There is no ETA for this.
Some information was being collated on a Bluwiki article here (created 22nd July 2008). This information was removed due to a legal claim by Apple that it violated the DMCA (first e-mail dated 10th November 2008). This is what happened next. Some analysis of Apple's claim can be read here (random laywer?) and here (Fred von Lohmann, EFF). The latest update is that the EFF has agreed to represent BluWiki.
There remains an IRC channel #ipodhash on Freenode to discuss the protection mechanism.