Millipedes like dark, damp places where there is organic matter. Centipedes also like dark, damp places but they feed on insects. Can you get a photograph of one, or check out images of each on Google to make an identification? Or did you notice if it was moving slowly (millipede) or fast (centipede)? If you only had one of them, and it came in the organic soil, maybe it was an isolated incidence. Did the bag of mix have a hole in it?
This article from Colorado State University Extension's entomologist might be helpful:
http://www.ext.colostate.edu/p...
