I have a question that I'm hoping you can help me with.  We have a
drop ceiling of just the white, thick drop ceiling tiles.  We had a
water leak which has left one discolored.  They are usually easily
replaceable, but this one has a board directly on the top of it,
preventing us from being able to push it up to replace it.  A friend
said that we could use bleach to take the brown stain out, but I can?t
imagine that would work.  Do you have any suggestions on how to get
rid of the ugly brown stain left from the leak, short of spray
painting a color of white that may or may not match?

You may want to try a rust-removing product that contains oxalic acid.

http://www.fabpro.com/products/spottingagents/rr.htm

http://www.chemspecworld.com/catalog_browse.asp?ictNbr=184