I just moved from Ft Collins, CO to Madison, WI. I've been drinking tea for years, and in CO I never saw any film on my tea. The first day that I moved out here I noticed the film, and it continues to appear on every cup that i drink (i've been here for a month). My tea also stains my cups out here, so i really have to scrub them. This had NEVER happened to me in CO, even after letting my darker teas sit in the cup overnight. In CO i used both regular tap and reverse osmosis water. Out here I do have a filter on my tap, and I've tried without the filter just in case.
The film doesn't seem to affect the taste, but it looks a little gross. My conclusion: it's something in the water. Dangerous? I doubt it. It's just visually...unpleasant, which somewhat affects the otherwise enjoyable tea-drinking experience. I drink several cups per day, so i don't really want to buy water... maybe I'll get used to it? If anyone comes across a filter that removes this, or figures out what it is, please pass it along.