I just had a brown ale that was way too sweet, so I poured a not yet carbonated beer into my sterilized french press, added 4 ounces of coarsely ground coffee - cold steeped for 48 hours, then just poured it right back into the keg.
Turned out fantastic - slight coffee taste, huge coffee aroma and bittered the beer to quite an enjoyable brown coffee ale

EDIT: The vodka method would still dilute taste (slightly) and is certainly less worry about infections... but I just wanted to cold steep beer, it's homebrewing, I had fun and it turned out great