This story begins before the first episode of Shoujo Kakumei Utena and spans the entire series. I would strongly advise against reading this story unless you've seen all 39 episodes.
I relied pretty heavily on the scripts and synopses at the Utena Encyclopedia to get the continuity right. (The last time I checked, in early January 2003, the Utena Encyclopedia seems to be gone from the web. If the site has merely moved, I would love for someone were to send me the new URL. Thanks!) The occasional dialogue quotations are derived from the scripts by the Utena Translation Project, although I generally rephrased things to suit my preference (without changing the meanings).
Oh, and where did I get this weirdo idea anyway? Well on the newsgroup alt.fan.utena, I made the offhand comment, "It has crossed my mind that the Dios who Utena sees is actually a projection generated by Anthy."
Verthandi replied, "Hmm . . . then that would liken her to the projector in Akio's planetarium!"
I responded, "Hmm . . . what if the planetarium is generating Dios, and Akio doesn't have as much control over the planetarium as he likes to believe? . . . Of course this would make the planetarium sentient. Hmm . . . that might be a fun fanfic. SKU from the planetarium's point of view."
Then Verthandi said, "Write it!"
D'oh!
"I, the Planetarium" was originally posted to alt.fan.utena in November, 2000.