Headboards can be done up in so many ways - you can simply drape a rug (like above) over an existing headboard or get out the power tools and start from scratch. I say that if it fits behind your bed then it's a headboard! Have a look at some DIY headboards by bloggers, they have all done a beautiful job and have posted their tutorials:
Happy At Home: headboard shelf tutorial - it's gorgeous
Mrs. This and That: DIY headboard - inspired by Pottery Barn
Classy Clutter: DIY upholstered tufted headboard tutorial - a whimsical take on tufting
Okie Chic: every girl loves ruffles - headboard slipcover!
Corner House: my master bedroom headboard tutorial - cornice box
Running On Happiness: DIY custom upholstered headboard - project tutorial
Look What Jeff Did: DIY headboard from table top - isn't that cool?