No doubt, as more intelligent anti-plagiarising software is developed there will be greater demand for auto-plagiarising software (that runs a thesauraus on every noun and verb and perhaps re-arranges sentences gramatically).

There's a fine line, however, between detecting plagiarising and matching on anything approximate.. being too clever can sometimes cause headaches.

At university I found it was mostly English-As-A-Second-Language students that cut and paste from other text books. So much it was painfully obvious to any native speaker of English (where text book language often differs from summary/report language).

I don't envy the exam marker's job. Imagine having to read essays anyway!