Not sure if the blog says anything about it (haven't read it) or if anyone cares, but I did a quick search, and discovered that the keyboard is actually a normal mini pda keyboard -- they just have an adapter for the USB to the iPhone connector (iPhone / iPod connectors have USB pinouts, so it's a straight conversion) and an OS X driver for the keyboard loaded. The driver would be the only thing that would require a jailbroken iPhone for this to work.
The keyboard appears to be this one:
http://www.palminfocenter.com/news/3003/review-minipda-keyboard/