To quote Google, "It's a logistics problem." We can only imagine. Trying to get every Android partner to follow a timeline for releases has to be a complete and utter nightmare, but the company seems certain that these stipulations won't cripple anyone's ability to innovate on their skins (or have too little time to make the needed changes).
We would've loved to hear a specific figure that we could start holding phone manufacturers to, but alas, it isn't to be. The only hard number thrown out today is 18 months. That's how long future hardware will be in the support cycle (at least, anyway), so you'll "soon" be able to count on your next Android device receiving all applicable updates for 1.5 years after purchase. As for phones that use custom skins, like Blur and Sense? Hard to say if that'll slow things down, and it's even harder to say if outfits like Dell will be joining this party at any time in the future. Though, to be fair, Andy Rubin clarified that there's an "open invitation" for anyone not listed to waltz on in. Naturally, we'll keep you abreast of any further developments from the show floor.