That is true for the Pi 3. On the Pi Zero, the bootROM in the BCM2835 needs updating, which is a revision to the chip. Possible, but we are all guessing about whether that is the one. It would have the side effect of bringing the booting of A+ and B+ into line with the Pi 3. The Pi 2 would need an equivalent change to its bootROM -- still possible, but less likely. (I guess that having made the change for the 2837 it is comparatively risk-free to port it to the others. And note Eben mentioned the chip revision numbers for all three -- or was that smoke and mirrors?)
The BOOTROM is in the VC4 VPU, which is common to all RPi SoC's, therefore it "could" be implemented in all future SoC Manufacturing.....
...but it may already be implemented in BCM2835 & BCM2836 SoC's manufactured after August 2015, as well as the BCM2837 ??
Unfortunately it can not be retrospectively implemented, so any new Firmware would need to check for the BOOTROM capabilities !!
Be great if USB BOOT was actually already in the BCM2835 SoC of the ZERO
I am going to stick my neck out and say: No. Not going to happen.
Yes, all three SoCs use the VC4. And Yes, all three SoCs have bootrom code. *However*, changing each of the other two SoCs would mean testing the code in simulation, making new masks, doing a test run of at least one wafer, verifying that the code worked correctly (or, at least, didn't prevent normal operation), going forward using the new masks.
Somebody would have to pay for the work to bring the older SoCs up to the same software level. Who is going to put up the money to do that for older designs?