Just broke when I opened it one day; from the looks of things it was a stress fracture of the plastic. However I'm not sure if it was the plastic itself going after lots of use (had mine for about a year and a half) or if it was something within the hinge sticking a bit too much. Its hard to tell as the spring within the hinge is quite powerful.

At least the Treo 600 doesn't have this design (although I've now got mine in a soft cover to protect the screen) and the cables I got to hook up the Treo 180 to my machine worked fine with the Treo 600 - not entirely unsurprising since they both have the same pinout on the external connector.