I downloaded the latest raspbian (that's 2014-01-07-wheezy-raspbian), dumped it on an SD card and the pi was running immediately. Also configured the I²C bus following the adafruit description. After attaching a PCF8574A to the bus, I could successfully detect it with

sudo i2cdetect -y 1

Resulting in

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 3f
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

0x3F is the PCF8574A's address if A0..A2 are tied high (which they were in my test). Fine.

I can now build the servo board that connects to the teensy 3.1.