AS/400 had some really cool technology built into it.
I have seen the binary translation in action: it was required when POWER6 machines came out. On a POWER5 machine, I remember it taking around 20 minutes for an application that was around 20 MB zipped, but for software licensing reasons not all of the server models exposed the full performance of the processor, so that might not be representative. And at any rate, like the OP said, it's a one time thing, and then the translated version is kept.
It does free them up in the future, they can make bigger hardware changes and just have to supply a bitstream player for the older code. I have a difficult time envisioning the architecture changes that make tons of sense to Apple right now though, other than like swapping video on their ARM chips and stuff like that. Running iOS apps on the desktop might make a ton of sense for some of them or some sort of tweener between the iPad and Mac book.
The native byte code was 64bit, though iirc the first implementation used a 32bit address. The result was you could ship an application once and when it came time to move architectures all you had to do is one command to retranslate the byte code.
Very neat. I'm not sure why this approach hasn't been used more often.
EDIT: Looks like they're going exactly the AS/400 route.