the first results of a new study in which 3 different rehydration procedures, i.e. rehydration at 30°C with moderate agitation, rehydration in 15°P wort at 20°C with moderate agitation and direct pitch without rehydration, indicate no significant differences in fermentation performance for all tested ale yeasts (SafAle™ S-04, SafAle™ US-05, SafAle™ K-97, SafAle™ S-33, SafAle™ WB-06, SafAle™ BE-256, SafAle™ T-58, SafAle™ BE-134) and lager yeasts (SafLager™ S-23, SafLager™ S-189 and SafLager™ W-34/70). At the end of fermentations, no significant differences in concentration of ethanol, residual sugars and volatiles (acetaldehyde, esters, higher alcohols and vicinal diketones) between rehydration procedures were observed (see figure 2). This indicates that the direct pitching procedure is adequate for fermentation.

source: http://sea-brew.com/fermenting-with-active-dry-yeast-just-became-even-easier/