Okay, "perpetual motion machine" might have been hyperbolic -- the comparison I had in mind was to what we might call a "weak" perpetual motion machine, which doesn't generate energy but is exactly frictionless, so it twirls forever without energy input.

So, don't get me wrong, it's a nice hack, but it's hardly perpetual or earth-shattering. One similar trick I know of is to have several credit cards, and use them to keep transferring the balance between them before interest accumulates; but this is less efficient, since the "free balance transfer" special offers occur relatively rarely.

Interesting! Didn't know about that variant.