I've always known Arial was a knock-off of Helvetica that Microsoft included to avoid paying license fees. That was "inside baseball" from about 15 years ago. I hadn't read font designer Mark Simonsen's excellent explanation of it from last year until now:

Now, Monotype was a respected type foundry with a glorious past and perhaps the idea of being associated with these "pirates" was unacceptable. So, instead, they found a loophole and devised an "original" design that just happens to share exactly the same proportions and weight as another typeface. (See "Monotype's Other 'Arials'") This, to my mind, is almost worse than an outright copy. A copy, it could be said, pays homage (if not license fees) to the original by its very existence. Arial, on the other hand, pretends to be different. It says, in effect "I'm not Helvetica. I don't even look like Helvetica!", but gladly steps into the same shoes. In fact, it has no other role.

Good reading if you want to know what's what.