This has been beaten to death, but here goes lol.
MS is not saying anything, and this is to keep a lid on things until necessary. With Mango they let the competition know too early and things were copied.
Now for the question: I would literally bet my right eye that Apollo will be distributed to all 2nd gen phones, and would almost do the same for first gen phones.
MS and Nokia both need this to succeed and they are NOT going to leave everybody in the dust with Apollo that already bought devices. ESPECIALLY NOT THEIR NEW HERO DEVICE (lumia 900) that JUST launched.
What will happen is simple. The update will be given to your current phone, and just like with ANY OTHER UPDATE, the only things it will NOT get is simply things the hardware limits of the phone itself allow. Just like with Mango, my Focus got the update, but things like Gyroscope & Front Facing Cameras don't magically appear in the phone, so those aren't supported obviously.
Everybody will get the OS update (can't say I know for sure on 1st gen devices, but don't see why they couldn't/wouldn't), and you will only be limited in the features that require specific hardware you do not have.