Thermostats can leech small amounts of current from the other 4 wires in the absence of a C wire (depending on the furnace), usually enough to operate on.
Yeah, I meant to comment on this, because it seems like the Nest needs the C wire. So I'm still confused about Tom's story. Oh well.
No, nest doesn't need it. It has an internal battery which is recharged by trickling from the aforementioned circuit.
In my case, it works perfectly (but it should as my boiler is what it was prototyped against); I only have a two wire hookup and being in a single story concrete slab house with no attic or crawlspace, pulling new wire would have been a nightmare.
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