I've seen complaints by others about the same
problem, however I dim my bulbs (~20 total)
down by 12% and 22% and they last for _years_
of daily dusk-to-dawn operation. (12% from
dusk until 11:30 PM, 22% thereafter until
dawn.)
I'm wondering if the filament design of some
brands of bulbs is such that the triac-
triggered AC waveform is causing them to
vibrate, which will kill a bulb in short
order.
My bulbs are mostly GE brand, a mix of 40-
100W standard bulbs and 25-40W flare tip
candelabra base bulbs.