I had this problem - it's fixed now but the kitchen ceiling still shows signs of water damage and is on my list of things to do...
Water had been flowing through unseen gaps in the tile grouting and dripping down behind the tiles. Discovered when we completely refurbished at great expense.
I tried searching for damp on the floor etc and tried sealing everything in sight (several times) but without success. When the tiles came off the wall the cause was obvious. I found damp and insect life!
Until it's fixed you can stop the drip into the room below by hand-showering - placing the shower head in the bath and picking it up when needed.
Test your tiles - spray a small area that usually has water splashes - have someone shout to you up the stairs.
Peccavi
October 2008