It's great to see that others are making serial port adaptors! I am tearing apart my place looking for my stash of Stealth ports and can't find them - which is driving me nuts because I saw them like a week ago when I was looking for something else! Which led me on a brief search because the electronics are not complicated. I was going to make my own a few years back but wasn't able to find out what the modem connector part is.
As for PowerBook use, I did test a Stealth in my Pismo (RIP) and it worked great, apart from (as CircuitBored noted) needing to install the jack somehow. I just left the cable hanging out the back of mine for like a month before I decided to remove the card. Didn't try my TiBook because it's delicate and a PITA to work on.
Definitely looking forward to the mini version! Also psyched to see that people are playing with the I2C and GPIO pins.