Okay I say each bulb does 10 gh.
That means 100 of them do 1th
100,000 do 1ph
This sounds a very reasonable estimate.
This would add one or two Watts to a 10 Watt LED or CFC bulb (0.1 J/gigahash). So the light bulb efficiency would decline by 10 to 15%.
The bulb would earn 1 or 2 cents a month from mining, assuming the light bulb was switched on 4 or so hours a day. Say 10 cents per year at current difficulty. Of course your electricity bill also goes up.
The vision of a light bulb mining was an efficient bit of crazy PR for 21 Inc. But totally unrealistic.
Building a Bitcoin miner, and associated Internet connection (WiFi?), into a light bulb is totally crazy. A light bulb makes light. A Bitcoin miner generates heat, not light. A miner built into a water heater could make sense as the heat is useful. A light bulb does not even make sense. Extra heat is just a sign of an inefficient bulb.