I just try to export on your new version of 1.8 questions from a category wich contains a calculated question and the export process STOPS and gives me the message:Unsupported question type detected in strange circumstances!
Effectively in the previous versions, the question header was exported with the "unknown" type but without any visual message during the export process showing that this question was exported as an incomplete question.
I would comment that your message could leave the user with a strange feeling that he has done a bad thing.
If your message mention that this question type is not actually supported by the XML format, I think it could be more useful to the user.
And perhaps, we could work on a process similar to the one on import i.e. letting the export continue with all the "Moodle XML supported" questions.
A more complex solution will be to allow the user to choose the questions for export as he chooses them for delete or moving. I had done such a proposal for the older showbank.php code but I am waiting for the new category management code before doing anything on this selection for export.
P.S. As users could want to import from previous versions, 1.8 version should be backward compatible and the"unknown" type should not generate an error but a warning.
The user is trying to import a file that he has obtained from a previous moodle version where this "unknown" type was the standard way to signal to the import process that this question was exported as an incomplete question.