Originally Posted by Cheapo
....if I were to start up a yeast batch in warm water.....
If you are talking about dry yeast, then yes you would generally re-hydrate in plain (sanitized) water. You generally don't make a starter with dry yeast. The cell count is so high, you have a lot of flexibility - you could just sprinkle it into the wort and be fine. But you'll probably have a shorter lag time if you rehydrate.
BYO did an article last year, testing the theory that rehydrating prior to pitching caused less stress to the yeast. IIRC, the theory was more or less confirmed but the difference was small.