Yes, the Smarthome Insteon switches used to have X10 capabilities in them (I read that they removed the X10 functionality in some of the newer revisions).
I have a bunch of them at my house, and they do work exactly as described. They transmit their own code with an on/off when they are turned on or off.
I was going to suggest the PowerFlash method (the PowerFlash takes an external low voltage - or dry contact closure, and sends X10 on/off commands on the powerline), triggering it with a wall-wart or something.
It would be neat to see a small module that had a small rechargeable battery or capacitor in it. When power was applied, it would send out an RF "on" signal. When power was removed, the circuit (powered by the charged battery or capacitor) would send out an "off" command before it died. A module like that would be great to act as a power-outage trigger (especially if there are other devices on a UPS that should be shut down - using a macro - in the event of a power outage).
Ideally, the "on/off alert" module would come in two versions - one that plugs into a wall outlet, and one that screws into a light socket (with a pass-through for a bulb). It doesn't need to control anything - it simply sends a signal when power is applied / removed.
Unfortunately, such a device doesn't exist at the moment.