"We continue to work on models launched prior to September 2011, please check back often for the status of older devices."
My guess is the newest devices have an updated bootloader that they CAN unlock easily. I am betting they may even be stymied by the security they put in place on older devices. This series of events makes sense to me:
Spring 2011 - announce we're going to allow unlocking
Summer 2011 - update bootloader to be easily unlockable
Sept 2011 - start releasing updated loader on devices
Dec 2011 - final certification that this is all going to work OK and release to the public
2012 - figure out how to get around our own security on older devices
I suspect the key difference might be that HTC doesn't want to release the following type of instructions: "Tap settings, Tap Applications, Tap Development, Enable USB Debugging, now install this software on your PC, connect your phone via USB, be sure to un-mount your SD card first... etc. etc." I think they would prefer to release a one shot install fix and that to my knowledge hasn't been figured out yet. In the end it may just not be worth the effort for them to do on older devices as well.
"We continue to work on models launched prior to September 2011, please check back often for the status of older devices."
My guess is the newest devices have an updated bootloader that they CAN unlock easily. I am betting they may even be stymied by the security they put in place on older devices. This series of events makes sense to me:
Spring 2011 - announce we're going to allow unlocking
Summer 2011 - update bootloader to be easily unlockable
Sept 2011 - start releasing updated loader on devices
Dec 2011 - final certification that this is all going to work OK and release to the public
2012 - figure out how to get around our own security on older devices