I made a peach wheat a few weeks ago and added 3 pounds of pureed peaches from a can before I pitched the yeast. When it came time to try out my first brew i found that my beer didn't taste peachy at all.The only time I got ANY peach flavor is when I belched after drinking it.
It still turned out to be a fantastic brew... In fact, I had a couple friends tell me it was the best hefeweizen they have ever had. The other upshot was the 8.5%ABV from all the extra fructose.
The moral of this story... I'm a noob and you should always add your fruit after they yeast is done working in the primary.