Mosaic « Effects « Illustrator Tutorials

1. How to Create a Mosaic Portrait from a Photograph - Vectortuts+    vector.tutsplus.com

2. Mosaic Image Effect in Illustrator | Adobe Illustrator    webdesign.org

Though Illustrator has the same feature (Filter > Create > Object Mosaic) it produces vector mosaic with much worse quality. See mosaic image on the left that was created in Illustrator.