One-sided mattresses

Total rip-off tag.

pretty blatant move by big mattress to make people buy their product faster 

^ YES...they OWE the public a 3-sided mattress.

Planned obsolescence time cut in half! Someone alert FOM. 

... which is why I sleep on an all cotton futon

in a van?

In addition to "eggs a la FaceOnMars", another proposed invention of mine would be a "build it yourself" mattress (that can be flipped).   The outer shell would be kind of like a suitcase in that you'd be able to unzip the entire container and customize the order and type of the inner content "modules".   Of the modules, there'd be all sorts of options to select from:  foam, cotton, spring, hard surface, semi-hard, etc. ... all can be obtained in various thicknesses.

The advantage of this system would be that you'd not only be able to tailor it to your liking directly from the manufacture, but you'd be able to tweak it along the way to really dial it in.

in somebody else's bm'r

Your idea has some merit, but what if you design/build your own mattress and its horribly uncomfortable?

Shoes and mattresses should both be tested first-hand prior to purchase.

I agree about shoes and mattresses ... but it could be setup like Sleep Number does it with showrooms comprised of several "base model configurations" that can easily be adjusted on the fly.   In fact, Sleep Number systems are very closely related to this idea in that I believe they will ship out different foam inserts after purchase to augment their base model offerings ... so it's kind of a rip off of what they're kind of doing already, but opening up the composition of the mattress to more material options.

Mine has 4 sides.

Moebius mattress.

Mine is 2 sided It Cost 1200 Bucks and Is Comfortable As All Hell !

One sided mistress