A quick search popped up this thread. I recommend you read through the comments before trying it though; it would appear it's easy to go overboard on the coffee.
I brewed an Espresso Porter last year. I added 16oz. of espresso to the bottling bucket on bottling day. It turned out great! Just figure out how strong you want the flavor.
I added 24 oz of cold pressed coffee at bottling time to a True Brew Oaked Imperial Stout kit. After about 3 weeks in the bottles it came out great. I really like coffee though so it might not be for everyone.
I would add coffee to the secondary or at bottling, myself. If you add during primary fermentation, I'd be worried that it could have an adverse affect on the yeast or that you could lose some aroma to the CO2 output.
I'm planning on brewing this for my brother, doing the all grain version though.
http://***********/stories/issue/article/issues/258-januaryfebruary-2009/1911-founders-brewings-breakfast-stout-clone