Today's discovery, by chance, was that some cellphones/smartphones expose a "standard old fashioned" serial modem when connected over Bluetooth to a computer. This means that it is possible to interact with the phone using the standard and extended AT command set.
What for? Well, any Bluetooth enabled embedded system (Arduino + HC-05, Raspberry PI + USB-BT dongle, ...) can have access to wireless telephony services such as SMS, dialin/out and potentially GPRS/Internet. All this without the requirement of a special modem device and an extra bulky cable in between. And probably you already have a suitable device laying around.
Not all cell/smartphones support this mode. For example an Android 2.3.3 Samsung Galaxy S i9000 does, while a Windows Mobile 6.1 Samsung SGH-i780 does not. I have two more cellphones with Bluetooth to test.
I think this is a simple(r) way to enable remote control of embedded systems, even though a lot of experimentation is needed.
Edit: Nokia 6233 exposes a modem over Bluetooth. Hooray!