Based on its name I think it's used in Arch Linux packages (at least makepkg says it is generating an mtree file at some point during the build process IIRC). However it appears mtree (the tool) isn't packages, so perhaps it's only using the mtree specification format?
I think there is a linux tool that does the same thing but I can't remember what it is. I just used mtree on my linux hosts and it worked fine.
http://man.openbsd.org/mtree