OK, did a little more digging and looked at some signals.
First picture is uart output with nothing attached.
Second is with PL2303 RX hooked up to board TX, with short wire. This works and I get good serial output.
Third picture is of longer wire. For this test I added about 60cm of jumper wire to the connection.
It looks like the 1.8 to 3.3 volt converter becomes unstable at this point - my (software guy) guess is the bidirectional detection is unhappy with the capacitance/impedance of longer wire.
Looks like short wires are going to be required for reliable serial port connections.
