In very recently updated versions of both Moodle 1.8 and 1.9 it is impossible to export questions from any category other than Default.
I have several categories named, e.g. category one, category 2, etc.
In category one I have a number of questions.
From category one, I click on the Export button.
In the Export questions to file window, curiously, the dropdown list has reverted to Default choice. I click on it to select Category one and click on the drop-down File format list. No matter what export format I select, no matter if I have ticked the "to file" box or not, the questions in Category one will not be exported. The questions from Default category (in any!) will be exported instead.
This is a serious bug.