>     portable_path("/foo/bar") <- throws on Windows

Not sure why this would throw, what is the purpose of portable_path? "/foo/bar" is perfectly reasonable on Windows.

>This is also a way we could solve the whole problem of absolute paths.
>It's clear that "/foo" isn't an absolute native windows path.

This is not at all clear. I have and will contain to argue that "/foo" is an absolute windows path, since it does not respect the current directory. Also very important to me, this goes well with the URI definitions of absolute and relative, and it would be nice if the path class could support full URIs.

Glen