Over time Apple will shift APIs over to Swift, while those using Objective-C will be told by their bosses not to bother moving to Swift, and voila, MS lock in!
Only time will tell: Swift isn’t even a year old!
Obj-C/C++ is a mature language, with cross-platform support already. (If a GCC exists for the specified target, then you can use generic Obj-C)
For now, Swift is a proprietary language, and its features could change or break at any time.
I don’t expect Microsoft to support Swift for some time. If they decide to do so, they might even have to reverse-engineer the Swift runtime, Chinese-wall style.
I will even go so far as to say that Apple is the one trying to lock-in developers: Swift apps require iPhone OS 7+, and I don’t see Apple making Swift cross-platform any time soon.
FWIW, the fact that MS is even making such an effort, is rather unprecedented.