Thanks for pointing out how many more interrupts the 328P has -- WOOT. If those are usable without terrible Arduino S/W gymnastics, this will greatly simplify AC phase control. It appears that the http://www.powerswitchtail.com/Pages/PowerSSRTail.aspx
can do AC phase control (dimming) of a non-inductive load, provided one has a time reference for zero crossings. It might be possible to add in a more robust snubber, and use this thing to drive small inductive loads. Not certain, but may be worth a try.
I't not clear to me that this product has a UL listing. If anybody knows about that, either way, please post the details. The disclaimer makes me wonder about this.
For a zero-crossing reference pulse, one could buy one of their http://www.powerswitchtail.com/Pages/ZeroCrossTail.aspx
devices. However, it's pretty easy to make your own zero crossing detector, for much less. The http://arduino.cc/playground/Code/ACPhaseControl
example has a schematic for making a zero-cross reference -- and code to do a basic form of phase control/dimming. (That code has a delay loop in its ISR, so I don't like it so much. But it does do the job, albeit not elegantly.)
AC phase control is even easier if you use an AC (transformer-based) wallwart as the basis of your Arduino's power supply -- you can easily and safely get the zero-crossing pulse using an AC opto-isolator. Using transformer isolated/stepped down AC also lets you stay away from connecting your zero-cross detector directly to the AC_hot and AC_neutral to drive your phase control. Happy and safe.
Krodal -- thanks again for posting those helpful links; I appreciate your taking the time and initiative to do so.