dan: For an OS, you get capabilites from the kernel, or some appropriately delegated authority (which from hereon in I will describe as "a security service"). It's most certainly not "we haven't thought about it yet": there are many operating systems in use which use capabilities for security (EROS and Amoeba come to mind).
There is more to it, namely, how are capabilities implemented?
One method is to use unbreakable abstractions. Implement the capability inside kernel or security service, and your normal memory protection system should make the abstraction unbreakable.
The other option (used in Amoeba, because it's a distributed OS) is to use cryptography. It's slow. Enough said.
To the Advogato amateur mathematicians: For those interested in a more "open" MathWorld replacement, I've registered a project on SourceForge under the working title Principia Mathematica. The first mailing list, "principia-discuss" should be up in a little while. Please add yourself if you support the idea of an open encyclopaedic dictionary of the mathematical sciences.
There will be some interesting social and technical challenges. The most difficult technical challenge, as I see it, will be how to handle mathematical and diagrammatic content with a web-based interface in a form that's convenient for mathematicians. I have some thoughts on this (which involve converting between LaTeX, Lout and MathML amongst other things), but I'd like to talk about it first.