The advice I got from the head brewer at Weyerbacher was to add the fruit most of the way through fermentation. The rationale being that with only minimal fermentation left to go, most of the volatile aromatics/flavors won't get blown off by CO2, and the yeast will still be active and more likely to ferment out all the sugars in the fruit.
Recently bottled: English Barleywine
Fermenting: English Brown Ale (primary), Wild Ale (primary), KBS clone (secondary), 07 Vertical Epic Clone (secondary)
Next Up: either a Porter or an Oatmeal Stout