i haven't had much to do with fluids before, but whenever i used them i never did change them too much and they always showed up

things to check:
(just saw your edit while typing this reply)
check that the fluids have a material attached to them (i've never assigned new materials to fluids, but i assume it's the same as assigning materials to geometry)

also which renderer are you using?
Mental Ray? Software renderer? or Hardware renderer? or another one?
i've seen fluids show up when using mental ray, never tried with the others

if that fails then...

check if you are using render layers, if so then check that the fluids are in the render layers that are being rendered

if you aren't using render layers then check the render stats of the fluids

in the attributes panel somewhere in there is a tab for render stats, make sure that Primary Visiblity (or visible, can't remember which one) is checked on.

if all else fails... then maybe someone knows something i haven't mentioned...