Thanks for the responses. Torben's explanation sounds suitably mysterious for such a nebulous problem. But I guess misery loves company, because it's a relief to know at least one other person has seen the same thing.
I'm using HTMLArea straight out of the box, and I had assumed that it would inherit the character set of the page it's embedded in, which is iso-8859-1.
I didn't set up the server and can't change it myself, but I'll see if we can add the patches to AOLServer or switch to a different version. I'll also find out what character set Postgres is using.