Possibly the important thing is whether Draco thinks of them as being sentient (sapient) as he's casting the charm.
Yes, I originally thought of it as the most plausible explanation. But then, Harry's remark must also make Draco's Patronus ineffective, just as explanation of Patronus 2.0 would, which very likely isn't the case.
Update: This post has also been superseded - new comments belong in the latest thread.
The second thread has now also exceeded 500 comments, so after 42 chapters of MoR it's time for a new thread.
From the first thread: