One analyst for IDC, Shiv Bakhshi, gave a valiant try at defining a MID during a breakfast lecture at the conference.
Bakhshi always invokes humor in his talks, but his presentation was a serious one. After questioning, rhetorically, whether the world needs yet another device category, he gave these characteristics for a MID:
It is a "browser-centric pocketable mobile device that computes."
It is 4 to 7 inches long.
It can turn instantly on.