Thank you for bringing up the idea of having better support for the .7z format in Ubuntu.

The 7z format is in fact support by file-roller for quite some time but it does require the installation of the command lines utilities to work.

Having p7zip installed by default was discussed in 2008 and decided against since the format was not judged popular enough to justify the extra CD space it would use, that discussion happened on https://launchpad.net/bugs/282294

The issue is pretty much addressed in natty though since file-roller has been built with sessioninstaller support now which means it will ask you if you want to install "p7zip" when you try open an archive using that format: https://launchpad.net/ubuntu/+source/file-roller/2.32.1-0ubuntu2

The other suggestion made there was to add .7z support to the gvfs archive backend, gvfs is using libarchive though which has been designed to work on streams format and 7zip is not in that category so it doesn't seem likely that will happen. You can read http://code.google.com/p/libarchive/issues/detail?id=41 which is the upstream libarchive ticket about the issue

In summary it seems that the issue is mostly adressed in natty with the new file-roller version.