That said. I presume then that it's not impossible for people to add their own levels as they seem to be doing with Halo for the Xbox (might I add... in a BIG way!!)
Of course, this would need some kind of modded machine and probably excludes you from Live etc... but it seems entirely possible.
Possible yes, but non-trivial. There are three (well, two and a half) unreleased tools required to generate additional content. 1) the sxwad packer 2) the texture XPR bundler and 3) the debug version of CSX. The latter component isn't entirely necessary, but really *really* helpful -- the CSX debug binary is capable of generating Resource Definition Files (.rdf) which are header files used by the offline resource bundler to build the run-time .xpr file. Generating RDF files by hand is possible, but totally unpleasant to do for anything on the scale of a full level's worth of textures. The second thing the CSX debug binary has is filesystem instrumentation for building sxwads; without this, you'd have to hand-generate the sxwad file as well, which is even harder to do properly than the .rdf file.
AFAIK there are no plans to release any of these tools. Somebody might be able to reverse engineer the compression and header formats used for these, build a level, hand-generate .rdf files and build .xprs, but hand-generating an sxwad file is next to impossible without the filesystem instrumentation from the debug binary. Which can't be shipped Valve, Ritual, or Microsoft for obvious reasons (it isn't certified, it would introduce all sorts of security problems, it would make cheating possible, etc etc).
So it's *extremely* unlikely -- even with a modded xbox -- that you'll ever see non-blessed additional content.