It's not difficult to run Debian on ARM. The difficulty is getting access to your phone's hardware. HTC devices (like G1, HERO) are quite open to development, so it was a lot easier for the community to built boatloaders and gain access to other hardware in the phone. So, don't get your hopes up for the Droid yet. The Droid hasn't even been truely rooted yet. Fortunately HTC is upcoming with a Snapdragon Android device (and highres screen etc). Yes it is persistent because you run it from the SD card. And Android OS doesn't touch your SD card.