I think the butterfly can actually print out "hello world" instead of blinking an LED 
more gcc avr butterfly stuff here:
http://www.siwawi.arubi.uni-kl.de/avr_projects/
It looks pretty straight forward, though that does not imply that I think it is the right thing to do. Smiley is right in that multiple chip support can fracture the community.
I think the 169 is similar enough to the 168 that they can play well together, no through hole option though but a pre-assembled butterfly makes a real nice beginners package.
The 644 however adds so many pins and functions that don't benefit the 168 core that I don't see that fully integrating with the IDE without costing the beginners some headache as they wade through arduino.cc with article after article about this one specific chip and what it can do, when it does not even apply to their hardware.
Of course if you make it easy to use a butterfly, then you make assumptions about what accessories are at which port addresses. Which is awesome for beginners with a butterfly, but alienates the blink crowd as they wade through arduino article after article about temperature readings and piezo sound notes based on the butterfly addresses.
But I definitely like the concept of a "standard computing device", with a display of known capacity and some buttons at known port addresses. I've seen a few 168 based ones but the 169 built in lcd controller is nice and low power is a cool angle.
So it's a lot to think about anyway. Doing it is the easy part