Don't forget, once you do purchase the $200.00 developers kit, you can also purchase a hardware dev kit for $99. Insteon also make use of a compiled "SALad" program so one can customize their hardware device. This is exactly what simplehomenet have done. AFAIK one can transmit the SALad app across the powerline/rf to an installed device, reboot and viola, you have customized extra functionality. I'm planning on developing an INSTEON UIRT device via extended commands.
One thing I did notice - the RF remotes don't have status capability... yet...
It shouldn't be too long before other devices start coming out... I can see insteon Door Locks, Insteon wireless contact sensors/relays, There is a 3 speed (I think) bathroom fan already converted to insteon... it's on smarthome's site...
How bout a Inteon to Bluetooth Bridge? hehe