(before: the bookcase assembled)
(shelf from bookcase for base, sheet of wood for platform)
(waiting for glue to dry)
(making sure it's centered)
Here's how I reused the bookcase I made when living in my one-bedrm, to make a platform bed; using two shelves and four posts as the legs and a 4' x 8' piece of wood for the support platform.
I had two options with the legs; use the posts as they are which would make the bed, without the mattress, 13 1/2"H (12" post and two 3/4" pieces of wood) which was too high, or lay the posts on their sides making the bed 5 1/4"H which was too low. I decided to go somewhere in the middle and cut the posts down 6" for a height of 7 1/2". Luckily my landlord is working on the apt downstairs and cut down the posts for me saving me a trip to the hardware store.
For the platform, I bought a 4' x 8' plywood ($19.75 at Lowes) which I had 2' cut off to use as a large bulletin board in the office area. I laid the platform on the floor and measured out where the legs would go and lightly glue them into place. Admittedly, I didn't bother screwing the posts into place as the wood glue worked just fine.
Once dry, I turned the bed over and flipped the mattress into place, centering the mattress over the platform. For more detailed pics, check out my Flickr.