For a queen bed, buy 12 "LACK" side tables from IKEA - http://www.ikea.com/us/en/catalog/products/20011413 ($7.99 each as of 05/26/10). Each individual piece weighs just 10 lbs and measures (as packaged) 22" W x 2.125" H x 33.125" L. Though made of particleboard and therefore extremely light, I was told each table can support the weight of one adult standing on it. I was able to load and fit all twelve packages into my subcompact car quite easily.
Arrange the twelve tables side-by-side - three on one side, four on the other Each table is 21.625" x 21.625" so the final dimension of your platform bed will be 86.5" x 64.875" - which is slightly bigger than a standard queen size mattress of 80" x 60" but once the bed sheets and covers are on the mattress, I haven't found it to be a problem.
I thought I might need to duct tape or otherwise tie the legs together but haven't needed to do it - the tables are totally staying in place on their own.
That's it - you now have your off-the-shelf platform bed for around $100 that you can take apart, move (even up and down flights of stairs) and put back together all by yourself at any time - no messing around with tape measures, hacksaws, drills, clamps and hammers in the garage for hours on end, no sweat and grime at all.
P.S.: I am also using an improvised mattress made out of eight $9 sleeping bags inserted into a zippered mattress cover to go with this DIY platform bed for a complete, inexpensive, portable, lightweight bedding solution. Details are at https://www.instructables.com/id/DIY-Mattress-Substitute-Portable-Inexpensive-L/