Yes, it supports I2S.
In fact, it supports stereo I2S input, stereo I2S output, mono ADC input, mono DAC output, and mono PWM output... all 7 I/O streams running simultaneously.
This web-based design is the easiest way to get a feel for the audio capability.
http://www.pjrc.com/teensy/gui/
You can drop lots of those objects onto the canvas, not merely to get I2S in and out of the chip, but to create a 44.1 kHz 16 bit streaming application that all runs while your Arduino sketch runs and controls everything.