This is on my never-ending list of things to look at. Jan Wieck changed the driver to stuff the error message into the right place in the struct used to communicate with AOLserver, but it doesn't seem to be returned to the Tcl db command. It's possible he didn't get it right or isn't doing it in the DML case.
I plan to spend much of summer working on PG and OpenACS issues like this, but summer's still about 4-5 weeks away on my (work) calendar.