I have at least a good half decade--maybe more--until I become a stadium-chair-toting soccer mom.
And I hope that when I get there, these nice people, or someone, is making this into a real product by then. OhGizmo has a link to this designer briefcase-cum-folding-chair, which is a great way to disguise the fact that you're carrying a chair with you.
It's a regular hard-shell briefcase with nylon straps that make it attachable to something sturdy enough to hold your own weight. Then you can sit and enjoy. I should also mention it's an art project (from 2004) by Joan Korbes and Denis Oudendijk, so it's not an actual product, but anyone who's remotely enterprising or creative could make this for themselves.
My favorite scenario: Sitting in your portable briefcase chair and typing on your portable computer desk using the Hip Office. Classy.