According to the actual WD-40 website, after putting the search term "shower" in, it came up with the following 8 results (from over 2000 total) of uses for WD-40:
8 uses found.
* Shines shower doors
* Unclogs shower heads
* Lubricates shower door rollers
* Protects shower heads from rust
* Spray on shower doors to keep them from rusting
* Protects grout from mildew in outdoor shower
* Removes rust from shower chair wheels
* Drives moisture from wheel bearings in shower door wheels
So, while the ceiling isn't officially on there, I suppose you could, but like whizzer said, might depend on what its made of.