> I'm starting to think I'll stick with relays instead. You know
> where you are with a switch

Yes. Slow. If you're planning never to do anything more
sophisticated than on/off then a relay is OK. Notwithstanding
the size, cost and ultimate contact degradation of large relays
that is

But opto-driving triacs does work

http://home.clear.net.nz/pages/joecolquitt/txless.html

(last picture). You can also use small trigger transformers

> What sort of application would an optically driven triac be better
> than a relay in? I can think of dimmer circuits and motor-speed
> controllers, but that's all

Those would be two major uses. But there's more to it than
that. Switching mains with relays can be very noisy, electrically
speaking. Because of the uncertainty wrt the activation and
closure time of the contacts you have no control over at which
point in the cycle the actual switching will be done. This can
cause a lot of annoying EMF. If though you can precisely control
the switching point (for example zero-crossing) this can virtually
eliminate EMF and problems caused by current in-rush

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