The [`auto_explain` module](http://www.postgresql.org/docs/current/static/auto-explain.html) has `auto_explain.log_nested_statements` since at least 8.4, so that's one option. It's a bit heavyweight but I'm not aware of any other logging option that will record statements run inside PL/PgSQL functions and triggers.

See [this similar question](https://serverfault.com/q/20106/102814).