"Warm water-as opposed to wort or a sugar solution - is the
rehydration media because, for optimal health, the yeast need to
rehydrate quickly. Sigrid Gertsen-Briand of Lallemand likens yeast
reyhydration to an umbrella- you want the shriveled cell wall to "pop"
open and spring in to its natural shape, to slowly unfold. The
presence of sugars or other substances in the rehydration water slows
the flow of water in to the cells. Likewise, until the yeast cells
rehydrate and get their bearing, they are unable to regulate what
passes through their cell membrane very well. Wort contains substances
that are beneficial to yeast but also substances that are toxic to
yeast. Healthy yeast cells don't take these substances in. However,
yeast cells being rehydrated may let some in while they're still
"stunned" from being rehydrated. Once the 15 minutes have passed, the
yeast are inflated and ready to pitch" If you would like to read the
rest of the quote it's in the newest BYO article Jan/Feb issue. It's a
very good article, lots of info on liquid and dry yeast.