"Write once, run anywhere" is a phrase that historically has referred to multiple platforms by multiple vendors--that is, platforms other than ones maintained by a single company. That is, not just Macs and iOS. Even if they do unify their development interface for all of their platforms, that still will be far from WORA, since I don't see them coming up with an easy way to port their apps to Android or Windows or Ubuntu anytime soon.

While you didn't use that phrase yourself, the implication that it could be applied to Apple's offerings reeks of co-option to me--I think Gruber's right about that, since Apple appears to focus on providing the best experience for each individual device (and devices from other vendors aren't even an issue for them--why should they be?). That said, however, I think you're right in that it would certainly be sensible from their perspective to make it easier for developers of one of their platforms to pick up development for their other ones.