Until recently there were only two methods for effectively automating outdoor lights. The first, replace your existing outlets or switches with X10-controllable versions, is the cleanest, but also the most expensive and labor-intensive approach.
The second method is to throw caution to the wind and use indoor-rated X10 modules with the best weather-proofing that you can manage. This actually works pretty well, and there are lots of approaches to make the modules last longer and remain safe, while exposed to the elements. If you're fortunate enough to live in a mild climate like California, its said that a sturdy ZipLock bag that's elevated from the ground (to avoid standing water) will protect a module for a couple of years or more. (See Hack #59 - Use Indoor Modules in the Great Outdoors in Smart Home Hacks for more discussion of this and other ideas.)
But if you aren't comfortable with either approach, a new home automation kit from Black & Decker provides what looks like an even better solution. Their Freewire Outdoor Light Starter Kit includes two very interesting lamp modules that are molded directly into an extension cord. In other words, the X10 "brains" (if you'll pardon the expression) are part of an outdoor-rated power cord. What a great idea.
The kit comes with more than just the lamp modules, and you'll be able to press the extras into service for your inside lights or other project. But the real gems here are the corded modules; use them for any type of light that you need to control, and with the appropriate cautions, appliances such as fountains, radios, and mechanized snowmen.