We all know that aeration is necessary for yeast growth. Does it facilitate more than that? If you massively overpitch, on a previous yeast-cake for example, do you need the same amount of dissolved O2 as a "normal" pitch? Many pro brewers rack a new batch onto the previous batch's yeast cake. Are they on to something?
My simple perspective on this one is that the oxygenation of the wort provides a catalyst for aerobic respiration stage in the yeast during which they strengthen the cell wall and generally make them healthy, happy and reproductive. Clearly if you are over-pitched, you are less concerned about the reproductive characteristic but you still want the happy and healthy part before they switch into anaerobic mode for alcohol and CO2 production as a survival instinct. i.e. I would think that the yeast cake contains mostly exhausted dormant yeast cells that need to rejuventate before they are thrown back into anaerobic survival mode. Therefore, oxygenation would be just as critical to a fully attenuated fermentation as with a fresh starter.
I've been wondering about this too. I'm planning on pitching an Imperial Stout batch onto a yeast cake from a pale ale that I brewed a couple weeks ago.
From my understanding of the subject I would agree with Jim Denmark that aeration/oxygenation is still important to help build up the yeast cell walls. And Brad's experimental study link is a good one to check out for some thoughts on ester production. High levels of oxygen in the wort and overpitching can both lead to decreased ester production--which would help the flavor in many cases but detract from it in others in which a high level of esters is desirable (such as wheat beers and many Belgian styles).
At any rate, not too long ago I chatted with a few people who frequently pitch onto yeast cakes, and they weren't at all concerned about aeration beyond that which occurs when you pour the wort in. Personally though, I'm going to oxygenate the wort in the kettle before I pitch onto the yeast cake just to be safe--I'm more concerned about having healthy yeast that can survive in higher alcohol levels than I am about ester production for this style.
At any rate, I'd like to hear more about what others have to say on this since I'm going to be using this technique next week.
Here is an experiment that aimed to determine the influence of pitching rate and oxygenation on ester production using a Weissbier yeast. The results were deemed "inconclusive."