<p>Is it possible to build a .dmg file (for distributing apps) from a non-Mac platform?
And if yes, how?</p>

If you're distributing Mac apps, then surely you have a Mac to write and test them. Why not simply use that same Mac to create the disk image?
[Edit] Alternatively, if you're distributing a portable app, for example a Java .jar file, why bother with a disk image? Macs understand .zip and .tar.gz archives just fine.
I guess what I'm getting at is, I don't understand how one might need a DMG disk image, but not have a Mac with which to create it.