There seems to be hundreds of tutorials and ideas on how to repurpose an old headboard into a bench, but as they are all so different, I'm quite keen on the idea myself!
The great thing about 'how-to' tutorials is that they give you a basic structure and help with the technical aspects of the build, but there is so much scope for you to do it your way. Take this idea as an example- a bench from a headboard sounds like there would be a one-rule-for-all kind of system but, as headboards come in all shapes and sizes, your starting point will be unique and so there is no reason your whole project can't be!
This one is from Mallie + Posh- a clean and delicate structure with fun to be had in choosing fabrics for all the cushions! Interior designer Mallorie Jones has posted a simple set of instructions which are great as guideline. Find them here.
Moving on to The Lettered Cottage for a more in-depth tutorial!
Kevin at The Lettered Cottage has posted a step-by-step account of how he made a rather robust structure for the bench. There is even a little video showing how it was done. Find it all here.
And just one more link before you are able to construct one of these blindfolded! eHow Home have another list of instructions for construction here. Sadly they don't have any images but I suppose that will allow you to use your imagination!
I'm keeping my eyes peeled for an interesting, abandoned headboard now! Watch this space...