"It seems to me there must be something happening with the wiring in the outlet where every fixture fails. Initially they work fine, but overnight they stop working completely."

Sound like a sensor that is not compatible with a CFL.

Just about every dusk-to-dawn sensor uses a solid state Triac to electronically turn the light on and off.

Triacs distort the voltage wave, even when they are fully on.
This can play havoc with CFLs that use electronics to operate the bulb.

Try a CFL bulb that is rated to operate with a dimmer and you might have better luck (they are deigned to cope with the distorted voltage).

One other thing that can cause any florescent light to fail quickly is inadequate grounding.

It makes it very hard to start up the arc inside the lamp, and can result in prolonged and repeated starting cycles.

I agree with brickeye, the fluorescent lamps are raising cain with your relays. I use an X-10 wall switch to automatically switch 7 CFL downlights around my house, and I had to use an "appliance-rated" switch, which has a third wire for ground (or something like that ... I'm not an electrician). If you look at the ratings of your sensors/fixtures, they will likely say something like "150W incandescent" or "150W resistive", which indicates that they won't play nice with CFLs.

""appliance-rated" switch,"

The appliance rating probably means the part has an actual relay and not an electronic (Thyristor) switch.

Correct ... it's definitely not a solid state relay, and makes a very noticeable thunk (sorry for the technical term) when it actuates.

"Correct ... it's definitely not a solid state relay, and makes a very noticeable thunk (sorry for the technical term) when it actuates."

Bingo.
No distortion of the voltage wave.

A thyristor takes about 0.7 V to start conducting (or ore).

That means the current cannot flow until the voltage hits at least 0.7 V (sometimes higher).
the sudden step in the current when the voltage hits the threshold (it is actually called that) prodecues a lot of heating in any transformer not deigned to deal with it.
the changes to deal with the current steps cost more, and in the market, cash is king and lower prices always sell better.