You are actually correct. Sid is not rolling. It is never technically put into freeze like Testing before release, but since Unstable's main purpose is to act as a staging area for Testing, it is effectively frozen at the same time as Testing. Unstable may have slightly newer versions of some packages during a freeze, or even packages that were removed from Testing because they were deemed too unstable or buggy for release, but no major new software versions are introduced. (For example, GNOME 3.4 is in both Sid and Wheezy even though GNOME 3.6 is considered stable upstream.) Once Wheezy is released, Sid will be forked to become the basis for Jesse, then it will effectively be unfrozen.
There are actually several proposals to create a truly rolling branch of Debian that are often brought up around release time. The consensus, at the moment at least, seems to be that creating a rolling release would distract developers from fixing release-critical bugs - thus preventing Testing from being released in a timely manner - and generally reduce the quality of Debian as a whole.
Canonical might be able to pull off a rolling release of Ubuntu with some software that they maintain internally, such as Unity or the Linux kernel, but they don't have the resources to independently maintain every package in the Debian archive. I think that their "rolling release" will focus mainly on a few key packages that Canonical themselves maintain, and the remainder will be maintained by regular imports from Sid. Effectively, Ubuntu's rolling release will semi-freeze the same way Sid does, though possibly to a lesser extent, when Debian freezes Testing. The only practical way around that limitation would be for Debian to create a true rolling-release branch, which isn't likely to happen, or Canonical to re-base on another distribution, which is even less likely to happen.