If you are using Linux Mint Gnome or Linux Mint Debian Edition, you will notice that Ctrl+G or Mark All Upgrades is disabled. This has been done to prevent users from installing upgrades that have not been tested and verified to work by Linux Mint team. Read more in this discussion.
Linux Mint Debian Edition is a rolling release distro based on Debian Testing. It is currently only available in 32bit and not for other architectures.
UPDATE: Rick Spencer, Ubuntu Desktop Development Team Lead, wrote in his blog:
Ubuntu is not changing to a rolling release. We are confident that our customers, partners, and the FLOSS ecosystem are well served by our current release cadence. What the article was probably referring to was the possibility of making it easier for developers to use cutting edge versions of certain software packages on Ubuntu.This is a wide-ranging project that we will continue to pursue through our normal planning processes.