Kyle wrote:
(snip)
The paint over the tub has started to slough in two spots near the
ceiling, and there is a stain on the ceiling directly over where one
would stand in the shower that looks like a water stain from upstairs
but there's no water or sewage line in the ceiling at that point.

I suspect they painted flat over semi-gloss, which is why the paint is
sloughing. I am planning on re-painting this bathroom with designated
bathroom paint - am I going to run into problems? Will my new, good
paint end up sloughing because of the crappy flat paint underneath?
Any thoughts as to what I should do about what looks like an old water
stain on the ceiling, to ensure that won't show through the new paint?

The only things I'd add to Steve's reply (down thread) is to look at
these primers when you get to that point in the process.
Peel Stop
http://www.zinsser.com/product_detail.asp?ProductID=66
Gardz
http://www.zinsser.com/product_detail.asp?ProductID=27

You should still do the scraping/sanding/washing to get the best surface
you can before you prime.

Make sure there is a good working exhaust fan and that it is properly
vented to the outdoors. Lack of proper ventilation could explain the
apparent water stain you mention.