the triac option seems to be looking like the most beneficial way so far...
If you use your own TRIAC, make sure to use an opto-isolator. Solid State Relays have an opto-isolator built-in. For your safety as well as the safety of the Arduino, there should be no electrical connection between the AC voltage and the Arduino. If you do that, you are essentially building your own solid state relay. There [u]special opto-isolators[/u] made to work with TRIACs
i would like to have a thermistor read the temperature of a reptile enclosure and display on an LCD panel of some kind,
You don't need to "dim" a heater. A relay is fine if you are not trying to dim the light. Temperature doesn't change instantly, so normally you turn the heat on until you reach the target temperature, then you turn it off. I'm sure you've noticed that's how the heater/furnace in your home works.
In fact, the delay makes it very tricky to "dim" or linearly-control heat. Imagine trying to drive down the road with a delay in the steering... You might turn too far, or not far enough, and by the time you realize you're going off the road it's too late...