I have this exact same problem - no easy solution yet, unless you're willing to buy something.
Check out the PowertailSSR I think it's called - you plug something into it like normal, then wire the Arduino into the SSR (solid state relay), and bam, there you go.
I might do it, but I wanted a do-it-yourself solution, so I've stalled while I work out the colour-changing LED lights - I think a coloured dawn might be neater. But I'm going to try be doing the AC version again too - it's just too handy being able to work with AC and DC.
For what it's worth, I think the triac's and Opto-Isolator is the next best solution, and probably what I'll do.
The guy at the electronics shop also told me that, since I'm just trying to dim an incandescent light bulb, I could also just run it with high-voltage DC, which is also something I'm considering, although it seems more practical to learn how to use TRIACS and Opto-Isolator. Opto-Isolator is also the coolest idea ever, so it has that going for it.
Good luck!