I have a similar problem w/ one particular light fixture on a "porch" blowing out light bulbs. I've been trying to figure out why, so I did the next logical thing: get online. The fixture is on the outside overhand porch of an outdoor restroom. On the inside, both the men's and the women's have flourecent fixtures, as well as an exhaust fan on each side. I just replaced the photocell which controls the porch fixture which is blowing out the bulbs. Before that, the light wouldn't come on at all - so, that now at least lets the fixture come on every evening through the night until sunrise the next morning. But now, the bulb(s) both get blown out after only being in there about a week; and, when I say blown out I mean blown out - one of the two bulbs actually get a whole about an inch in diameter blown out of the glass bulb. It doesn't just blow the filiment out inside the bulb, it blows a whole in the glass. And, its done it twice now. Hence, my prediciment: Why is the bulb blowing out? It should last 6 months or longer since it is turned on and off only 1 time/day and just runs at night.

Is it because it is on the same circuit as the fourecent fixtures inside? Do the ballasts in the flourecent fixtures cause surges which are blowing out the incondecent bulb under the porch outside?

Puzzled!

If this circuit is on a circuit that shares a Neutral with another circuit, you may have a loose neutral.
Do you notice any bulbs getting extra bright or dim?

You might have a water issue being the fixture is outside or a bad socket in the fixture. Have you replaced the fixture with a new one, if not I recommend it for a start.