A PDM output microphone, such as the ADMP421 or ADMP521, cannot directly interface to an I2S port. Theses microphones should be connected to the PDM input on codecs such as the ADAU1361 and ADAU1761. The ADMP441 MEMS microphone has an I2S output that can be connected directly to a processor or codec's I2S input port.
We have an Application Note and a few Circuits from the Lab that explain connections between digital microphones and other devices:
AN-1323: High Performance Digital MEMS Microphone Simple Interface to SigmaDSP Audio Codec
CN0208: High Performance MEMS Digital Microphone's Simple Interface to SigmaDSP Audio Processor (ADMP441, ADAU1446)
CN0266: High Performance Digital MEMS Microphone Standard Digital Audio Interface to Blackfin DSP (ADMP441, ADSP-BF527 Blackfin)
Updating this post.
Since this FAQ was written many years ago we have developed some parts to translate PDM to I2S with a very low cost part. The ADAU7112 and the ADAU7118 are both PDM to I2S converters. The 7112 is a stereo part and the 7118 can take up to 8 audio channel inputs. Two channels per PDM data stream.
Also, there are several new parts that can take a PDM data input directly. Here is a list: