Black and Decker makes an outdoor extension cord with an X10 receiver in it just made for these circumstances. I've seen it on Amazon:
http://www.amazon.com/Black-Decker-FWLROD-Freewire-Receiver/dp/B0001VO36K/ref=sr_1_fkmr2_2?ie=UTF8&qid=1278430090&sr=8-2-fkmr2
It's called the FWLROD - Freewire Outdoor Lamp Receiver
I have one of these units outside on my deck -- and a spare I bought at the same time from Amazon sitting in a drawer. During the summer, I use it and AHP to control a small fountain. Around Christmas, it controls about 1,000 LED Christmas lights as well. The FWLROD works very well and handles our extremely cold and snowy winters without issue. I highly recommend the FWLROD - Freewire Outdoor Lamp Receiver for your outdoor X10 uses.
BTW, I've read about the issue of dimly glowing LED's when off with this and other X10 units and fully expected I'd have the same issue with my LED Christmas lights. I thought I'd have to add a string of conventional bulbs to the mix to solve it. But, mine don't glow when off. I thought it was because there are so many but I tried it with just one string and it works fine too. I'm not sure why this is happening. Any ideas? The lights are just over a year old -- if that makes any difference. For comparison, I temporarily hooked up a small string of other (older) LED's I have to an X10 appliance module and they DO dimly glow when the unit is off.
Anyway, the FWLROD works great and I recommend it.