I was able to spend a little time with the ID dosbox release, and confirm that it is, in fact, a modified derivitive work. It has apparently been linked with the valve steam library. In addition, some of the strings appear to have been obfuscated, indicating some form of packer/obfuscating system.
Copying dosbox.exe to a seperate directory will result in SDL errors. Using a stock SDL, it complains about being unable to load steam. As such, it is a modified version.
Valve would be legally obligated at this point to open-source (under the terms of the GPL) the steam stub against which this was linked. Failing this, they would be committing copyright infringement for commercial gain. Depending on what a court thought about the level of integration present, there may technically be a case for this being a derivitive work of steam and dosbox.
Statutory damages of $150,000 per offense suck, but I would imagine that they would pay them before they open-sourced steam. OTOH, it's quite possible that a court would determine that the level of integration wasn't sufficient to impose a burden to distribute the Steam source code, and then they would have to decide whether or not it was worth it to open-source the stub.
You may want to contact the Software Freedom Law Center. They handle the GPL violation stuff for busybox.