Well, being a complete freak I plugged a power cable on the serial port of one of my C1000 and, well, it didn't work.
As a matter of fact, I've also plugged the Zaurus to a 5V 1A USB charger (i.e in the mini-AB port) and - well, didn't work either (so I have cut the USB plug of the charger and replaced it with the power plug from the Sharp charger, since it's damn 110V - now it works, hehe).
I don't recall well, but I think I've tried a USB-serial cable too and, guess what - it didn't work.
Conclusion : it's TTL level only, whereas other models weren't. But it's fine with ttyS0