I don't really follow Mono, but I've been seeing tweets tonight regarding support in NuPack for mono being worked on and changes being pushed to CodePlex throughout the day.

http://nupack.codeplex.com/SourceControl/list/changesets

it is open source, so I believe support could be put into mono etc.

"the fact that it requires windows and visual studio"

Isn't that what .NET is all about?