Check out this series of interviews with brewing expert, Eric Watson, on Beer Tools:
http://www.beertools.com/html/articles.php?view=245
Here's Eric's surprising advice on aerating wort:
"If possible, don't! The reason is that it is not the wort that needs the oxygen, it is the yeast. By oxgenating the wort instead of the yeast starter, it will cause an over production of cells due to the excessive oxygen presence. This then leads to the production of unwanted esters and higher alcohols that will compromise beer flavor.
When oxygenating starters, you cannot use pure O2... the reason is that the uptake occurs too fast and without a dissolved O2 meter ($$$), you cannot tell when to stop. The way to properly do this one is to aerate using a high pressure aquarium pump, sterile air filter and a stainless steel aeration stone, all of which are redily available. It is virtually impossible to over-aerate using air, so you will avoid oxygen toxicity problems that will occur if trying to do this with pure oxygen."
When using dried yeast, he recommends no aeration at all. The yeast was properly aerated before drying and has been properly prepared for the fermentation immediately. Just rehydrate the yeast (he recommends 90deg water) for 30 minutes and dump into the non-aerated wort.
That's some wild and crazy stuff, huh?!!!
My personal experience backs up the oxygen toxicity problem. I used to pitch the yeast then aerate with pure O2 for several minutes. The result was huge lag times and slow ferments.