Turn a mains light on slowly

The basic circuit is pretty simple. C1 charges through R1 and turns on the triac when the threshold voltage of the diac is reached.

^{simulate this circuit – Schematic created using CircuitLab}

Now you've got the choice over how you vary that resistance with time. ThreePhaseEel mentioned the excellent idea of using a Negative Temperature Coefficient resistor (aka the common thermistor), tied somewhere near the lamp. Note though, that the lamp will not light up linearly with time as NTCs' resistanceVStemp is a decreasing exponential. This might be a problem.

The first thing that pops into my mind would be a 555 timer in astable configuration supplying a clock to a 555 in PWM configuration, with the modulation input being the charge of a capacitor (either through constant current or within one time constant to make it linear). The output of the PWM 555 could be either optocoupled to a MOSFET or directly tied to a solid state relay. However this sounds like overkill. If you prefer simplicity over cost effectiveness, you could replace the 555 with a microcontroller, which gets everything down to wall DC adapter-uC-optocoupler-gate resistor-MOSFET. Still, sounds like overkill.

I personally like ThreePhaseEel's solution, and I'd like to hear that someone else has an idea as simple that makes the lamp light up linearly.