I've come across a workaround that does exactly what I want, but jeez its clumsy and inconvenient...
Add each of the following to your contacts:
Name: Divert [here] if busy
Name: Divert [here] if unanswered
Name: Divert [here] if unreachable
Name: Divert all calls [here]
...where 0x = state area code and xxxxxxxx = desired phone number
Name: Cancel all diverts
Name: Cancel Divert if busy
Name: Cancel Divert if unanswered
name:Cancel Divert if unreachable
The "Cancel All Diverts" technically could be the only cancelling entry required, but if you run multiple diverts, thus adding a further layer of Conditions, then you will cancel all these conditions as well by using the blanket "Cancel All"
It would be relatively smple to write an app that would add options to set the above Conditional Call Forwarding within the menu of Settings-Phone-Calls, but the Apple iTunes EULA does not allow apps that change or improve the functionality of the phone's operating system, so such an app would have to be made available only through a 3rd party vendor, which thus restricts the apps to working only on jailbroken iPhones.
Apple, you make some nice kickass hardware, but your software policy sucks balls.
Edited by Antraman, 27 April 2011 - 09:09 PM.
Sometimes I think the surest sign that intelligent life exists elsewhere in the Universe is that none of it has tried to contact us...