Is it possible to build a .dmg file (for distributing apps) from a non-Mac platform? And if yes, how?
It does seem possible to create DMG files with some third party tools. A quick google search reveals at least a few commercial tools:
Not sure about any OSS/freeware options, but it does at least seem possible if you are so inclined.
Edit: I also forgot about MacDrive, which is another great tool for working with HFS+ filesystems under windows. Since a DMG is basically just a HFS+ filesystem snapshot, it is probably possible with MacDrive to create DMG's as well.