In versions of Minecraft before the current 1.0.0 release, you could use the Silk Touch enchantment to get Ice blocks, and break them in the Nether to produce water source blocks. However, this was patched, and they won't create water in the Nether anymore.
The only way to place water in the Nether, currently, is to place a CauldronCauldron and fill it with water. Sadly, it isn't as good as an infinite source, as it can only fill 3 Glass Bottles before needing to be refilled, but it's the only option for brewing in the Nether at the moment.