X10 is an older power-line carrier home automation technology. Indigo supports many different types of X10 devices. In many of the How-Tos on this site, you can use INSTEON and X10 modules interchangeably. Remember, however, that X10 modules don't repeat their signals, so you may find the technology less reliable. Adding X10 repeaters may help.
Most X10 devices do not support 2-Way communication: that is, it's not possible for the PLC (CM11, CM15, etc) to request the current status from the device and likewise when the device is manually operated, it doesn't broadcast to the PLC it's status. Therefore, it's impossible for Indigo to know/maintain state information for that device. Indigo can still control it, and when it sends a command it will assume that the device is in that state.
Note: We often get requests to upload to the CM11/CM12/CM15 - we have a philosophy about controllers running standalone (without the Mac running). The short answer is that we don't support upload to any of the current standalone controllers.
Here are the common X10 device categories that you'll find. Unless specified, all devices are one-way:
X10 Security Devices operate with a marginally more secure communications model, in that each device has an embedded identity that is logged by the security controller. Event communications from the X10 security devices to the SC9000 controller can be snooped by the Indigo Server, the Security Applescript can be used to translate these X10 Security Events into actionable information.
Do you have other X10 devices that others might find useful? Want to write up a How-To page to help others use it? Send an email to indigo DASH support AT perceptiveautomation DOT com and let us know that you'd like to contribute!