A few yards of fabric, pinned taut to the wall, is a beautiful and glam substitute to a wooden headboard.
Go with artwork! bright, graphic paintings energize an otherwise neutral space.
Search on Craigslist or at flea markets for fireplace mantels for a unique headboard alternative.
Try wallpaper! Have extra wallpaper lying around? Use it to define a headboard space behind the bed. For extra pizzazz, frame the wallpaper with wood trim.