amunoztico: Welcome to the forum and to X10.
You concerns about X10 security are legitimate, and unfortunately the answer is there is no security. You are correct, anyone could use another X10 remote and control your lights/appliances from outside your house.
However, X10 is not as widely used and one might think. The probability of your neighbor(s) having & using X10 is very low. The chances that someone will sit outside your house with a palmpad remote and try every house & unit code and watch for a result is even lower. You would most likely have more false turn on/off events from electrical noise than from someone outside your home.
X10 is not the most reliable system and should never be used in any situation where danger can occur if a unit gets turned on or off unintentionally. But as far as for controlling lights or appliances such as fans, then security shouldn't be too high of a concern. If you find your modules turning on or off unexpectedly (e.g. from a neighbor), using a different house code / unit code should solve the problem.