Howto enable disk mode on iPod Touch 2G.
Short answer: as of iPod Touch software 2.2.1 there is no built in way to do it. I guess this is because the OS on the Touch is a cut down version of OSX, while on regular iPods its not.
I've found two third party methods of having disk mode on your Touch (and iPhone as well): Phoneview $19.95
and Touchcopy at $24.99
Update: There's another application that make you iTouch / iPhone show as a disk: iphone explorer from www.iphone-explorer.com. Seems to work OK, and its free!
You can get even better value by installing Good Reader from www.goodiware.com/goodreader.html, currently $.99.