Apropos doorbell detection, there is a nice little Phidget, the 1135 - Precision Voltage Sensor
. This will detect A/C voltage up to 30v. Just place it across the doorbell and go (well, you do need a Phidget controller).
I have some exciting news I can now share. The Phidgets people are working on greatly improved OS X (10.6) support. The basic Phidgets libraries on OS X already support change Events (interrupts) from sensors. However, the existing AppleScripting extension does not. The Phidgets people are developing a new AppleScript interface (now in very limited beta) that allows the execution of arbitrary AppleScript handlers specific to a change event.
In the case of the doorbell detector, it is possible to have the doorbell press be detected by the voltage sensor and then have an AppleScript handler interact with Indigo to do something (or send a RESTful api call).
There is no release date set for this new software.
BTW, I believe the existing Python libraries already support this functionality. See Python
on this page
. With the upcoming plugin API, Phidgets could become full fledged Indigo devices!