I note that the Raspberry Pi folks are recommending the Raspbian fork of Debian for the Pi. This is the hard float port to ARM version 6 undertaken by Mike Thompson and Peter Green (who is also a Debian developer).
That being said, though, they have also released an armel copy that is fully binary compatible with Debian Wheezy for use with software that can't readily be rebuilt e.g. Oracle Java. This is understood to be slower than Raspbian but otherwise contains identical software.
The changes made by the Raspberry Pi folks are fairly minimalist: /opt contains some of the graphics example code which takes advantage of the non-free GPU and the raspi-config script under /usr/bin which allows initial configuration at first boot.
The /etc/apt/get/sources.list refers solely to Debian repositories and the software supplied by the Raspberry Pi project can be readily updated to latest Debian Wheezy. All in all a nice way to have a silent Debian machine in a tiny case.