My take of what each number should mean:
Major: Huge architectural changes or userspace stable API breakage. So 2->3 would mean something like, Fedora 15 would not even run correctly on Linux 3.0. This is what the kernel *currently* uses the minor number for. Evidence: Try running Linux 2.4.x on a reasonably modern distro. Hardware support aside, I don't think you would be successful in any capacity because so many features depend on 2.6 functionality. In fact, some features depend on at least a certain micro version of 2.6, like 2.6.8 or 2.6.18. And then some filesystems (ext4) didn't even exist in 2.4, and they've never (AFAIK) been backported.
Minor: Stable releases representing significant feature development, but evolutionary. This is what the kernel *currently* uses the micro number for.
Micro: Bugfix-only maintenance of a minor release. This is what the current *currently* uses the nano number for.
Nano: Distros that have their own patch series should use the nano number, followed by the distro name. Making this somewhat standard would help with version tracking. And no, I absolutely hate the dash that all the distros tend to put after the micro number as it stands. Don't write "2.6.32-78-ubuntu1"; write "220.127.116.11ubuntu" (that would be the third-generation kernel, the first significant evolutionary release, the second maintenance release of 3.1, and the 78th patch release of said kernel by the Ubuntu kernel team. If the kernel team insists on keeping the same patch release and making a patch or fix to the patch (or to the package build), just tack on a number after the distro name, as most distros currently do.