Originally Posted by masskrug
I've done both and have not seen any difference. Save yourself some time and pitch it dry on some well aerated wort.
^^^ This. Experimented with dry vs rehydrating on a commercial scale and came to the conclusion that there was no appreciable difference. Since it is a) an extra step and, therefore, b) another potential source of contamination, I simply don't bother with rehydrating.
It is true that a large number of cells won't survive the pitch, BUT the ones that do survive feast on the weak... so to speak. I pitch 1 pack into 5 gallons and typically see signs of fermentation in around 8 hours. No problems.
Also, dry yeast includes sufficient lipids for growth so that aeration is not actually necessary... I aerate anyway, but don't get extreme.. just let the wort splash and churn as I pour it into my ferm...