A carrier grade Linux system has somewhat different requirements than Linux for Web or file servers, or Linux on the desktop. One of the primary requirements for carrier grade systems is real-time support, which is typically not important for mainstream Linux use. The CGL specification also requires compliance with the Linux Standard Base (LSB), Service Availability (SA) Forum Hardware Platform Interface (HPI), and POSIX compliance.
Debian is not yet certified for the most recent CGL specification. The CGL 3.2 specification adds in areas like security and clustering, which are not available in stock Debian -- but it's possible that Debian can be brought up to spec in these areas as well.
When CGL was first conceived, this set of requirements was much harder to achieve than it is now. Bill Weinberg, senior technology analyst for OSDL, points out that "much of the technology was not present in mainstream Linux" around the time when the 2.6 kernel was in development. "In that timeframe both distribution suppliers and OEMs would have to go out and collect the components for a CGL distribution.... At least half of CGL requirements have found their way into the mainstream today." He says that, while it's still not easy to put together a CGL distribution, it is easier.
One example given by Weinberg is POSIX threading. Linux initially had its own threading model that was not POSIX-compliant, but it has been modified to be POSIX-compliant, and Weinberg says that "carrier grade, as a group, was instrumental in bringing that forward."
Weinberg says 16 companies have announced CGL deployments, including British Telecom, France Telecom, and NTT. It's also likely that other companies are using CGL but not discussing it publicly.
Of course, Debian is already being used in carrier grade systems -- the official registration just adds official recognition to Debian's status. Hewlett-Packard chose Debian years ago for CGL, mainly in order to be able to modify the OS more quickly than it would be able to if it had to go through Red Hat or Novell.
What Debian brings to the table
Weinberg notes that Debian gives vendors a unique point in the "continuum" for CGL. On the one end of the continuum, vendors can choose to create their own CGL distro from the open source components. On the other end, vendors can choose to use general purpose distros that meet CGL specifications, such as Red Hat or SUSE.
Debian, however, allows vendors to customize the distribution extensively while still using a distro that enjoys strong community support -- and takes much of the heavy lifting off of the vendor, who will need to take control of only the parts of the distribution it wishes to change.
CGL software development is a two-way street. Alan Meyer, HP's research and development manager for OSS, says that work done by carrier grade vendors and customers is being fed back into projects that are available to the entire open source community, such as implementations of the Stream Control Transmission Protocol (SCTP). Meyer says that HP is interested in pushing improvements back upstream, "so that anyone can benefit."
Meyer also points out that when companies like HP do standards testing for Debian, in order to meet customer requirements, that carries over to the distribution as a whole. Work that goes into seeing that Debian is standards-compliant benefits anyone who wants to do work in that area, not just the vendor and customer.
With Debian offering the components to meet CGL compliance, downstream distributions based on Debian should be able to become CGL certified fairly easily as well.