Although one could say you are staking Bitcoin when you run a node, I'm not sure I'd call it a Proof-Of-Stake system as the term is commonly understood. We're not replacing any consensus mechanism, or voting on anything by having more staked bitcoin. The way I see it, it's a way to facilitate bitcoin-denominated transaction velocity without incurring the on-chain costs. Staking keeps the symbol and the real accounted for.