The MIDI sync is actually detected by Live (the corresponding "LEDs" blink accordingly), but pressing play on the MIDI master computer doesn't make the MIDI slave play. Furthermore, when I turn the sync off, the slave computer stays locked and won't play unless I restart Live.
I became aware of this problem while I was teaching my Ableton Live class. Imagine what my face looked like when I said "oops, I think we've just found a bug!".
I tried this at home too. My hardware configs are on the sig.