Hi,
..Net Framework is not platform independent, but optimized for windows
systems.
There are ongoing efforts of porting .Net Framework to other systems, and
most commonly known is the Mono project, and through Mono you are able to
run .Net programs in Unix or other non windows operating systems.
However, if you plan to write .Net programs for non windows platforms, you
are best off starting with Mono instead of regular .Net as not all
libraries have been ported.
The url for the Mono project is
http://www.mono-project.com/ but at the
time of writing it did not respond. Until it comes back up, you may
settle for the Wikipedia entry
http://en.wikipedia.org/wiki/Mono_(software)