But I think the way Apple could most hurt Pebble is not by changing the SDK, but by releasing its own linked-to-your-iPhone wristwatch gadget. (Imagine, say, an iPod Nano with Pebble-like features and a LunaTik-style strap.)
I really don’t think the iPod-as-a-watch crowd is big enough for Apple to go after the market.
As it stands today, the iPod nano sucks as a watch. If Apple were to bake in deep intergration with iOS devices, I think the product would be much more compelling.