From Zalmoxsis:

I had some thoughts on the word 'amanuensis', too. However, as I tried to write these down, I found that the matter could not possibly be settled briefly.

In short, however -- comparatively short, that is, leaving out many fine points -- the role of amanuensis goes beyond that of mere secretary, but implies also either a servant-master relationship (which, in the ancient world, was very different from that of employee and employer nowadays); or, that one felt oneself indebted to, for example, one's teacher. Good students who were too poor to pay the fees often served their teachers in such a role; usually it meant also that one might be asked to gather firewood, fetch water, or do a little cooking. In other words, an amanuensis usually lived as a companion and general assistant to a teacher, but had to earn his keep.

Suetonius describes himself as amanuensis to the Emperor Hadrian, but here again it is clear that he is a servant to his master. And The Oxford English Dictionary says that the word is short for the phrase servus a manu, one who serves by his hand, which is to say, taking dictation, plus the suffix -ensis, or 'belonging to'.

A good contrast, by way of example, from the same period, is the relationship of Arrian to the Stoic philosopher Epictetus. Arrian is called the 'disciple' of Epictetus, and seems to have transcribed 'class notes' from lectures and talks, which he later published for the benefit of others; but he is never called an amanuensis, nor scribe, nor anything else that might indicate indebtedness or service on his part. Arrian was the wealthy son of a noble family, well educated, and would later be appointed governor of Cappadocia by the Emperor Hadrian; also, even though Arrian was a Greek, Hadrian gave him command of two legions, and he was a successful general. In later life, he retired to Athens to devote himself to literature, and published, among other books, a history of 'The Campaigns of Alexander', and an account of the teachings of Epictetus.

Epictetus was a former slave, and also lame; both of which would have barred him, for life, from any formal association with the nobility; Simplicius, furthermore, says that his only possessions were his bed, a cooking pot, and an earthenware lamp. However, despite these stigmas, nothing prevented their informal association. Arrian would have looked for lodgings among respectable persons, and is never mentioned as a companion to Epictetus. And no doubt he paid his way in cash. The only companions of Epicteus were said to be an orphan whom he had rescued, and an old woman whom he employed as the child's nursemaid. One imagines that he supported them from the fees he collected by teaching, for he had made a reputation as a philosopher, despite his low social status.

I hope that this clarifies just what the word amanuensis implied in the ancient world. It is determined partly by social status, relationship, and either obligations or indebtedness. Therefore, it is not merely a high-falutin' synonym for a secretary, but implies at least a servant, and very often a slave.

Late antiquity -- about the sixth century or so -- is probably the last time we find persons in the role of amanuensis; certainly, I believe, no later than sometime during the early Middle Ages. Any use of the word after that is a literary affectation.

That's a fascinating etymology and would have made for one hell of a footnote in a brief newspaper article on Sol LeWitt.  Seriously - I'm glad he sent it in, and maintain its usage in the context cited was pure PAW.

All that said, I did find a good usage of it at http://www.newsweek.com/id/167639/page/2:

The national press had largely stopped paying attention to McCain as his campaign spluttered in the spring and early summer. But on a trip to New Hampshire in mid-July, Mark Salter, McCain's closest adviser and all-purpose amanuensis (he co- authored McCain's bestselling books and wrote his speeches), noticed that the press corps had suddenly swollen, and not just with beat reporters.

Reading the whole article makes it clear that Salter is more than a secretary, but someone who travels with McCain and is responsible for translating his career, speeches and campaign into a narrative.  McCain doesn't dictate or even participate in the writing process - Salter uses McCain for the raw material, which he then uses to work into a literary shape.  That's an amanuensis.