I can confirm this works (now):
Beaglebone Black / Angstrom
Adafruit Logic Level Shifter http://www.adafruit.com/products/757
Attiny 84's and 85's - all the same bus (the 'third' bus, the second external). There are 6 mcu's, each is on about 1 meter of recycled ethernet cable, all tied back to headers on stripboard.
There are NO extra pullups; the shifter has 10k resistors on it, and I think what's happening is the BBB has internal pullups on the 3.3v side, which shows as a 5v pullup on the high side.
I can now get reliable i2cdetects, so I'm back on track...