And now you know.
From a note included with my water utility bill:
"What is that pink stuff on my showerhead?
Water utilities throughout the country have received questions about a mysterious pink film on plumbing fixtures. The main culprit is a naturally occurring airborne bacteria called Serratia marcescens, which produces a pink residue. S. marcescens does not come from your drinking water; its origin is usually soil or dust, and the bacteria is attracted to moist surfaces. The good news is that a common household cleaner, chlorine bleach, or similar product helps to curtail the onset of the bacteria. The best solution to keep these surfaces free from the bacterial film is continual cleaning and keeping them dry. In addition to being unsightly, S. marcescens has been linked in rare cases to urinary tract infections, wound infections, and pneumonia."
Okay then. I actually was curious about that....