Scilab is available as a binary package. However, we are working closely with GNU/Linux & *BSD distributions to create better packages. If you are a packager and you have some questions, need to commit patches upstream or remarks, please contact the Scilab Dev Mailing List.
Debian is an non profit organisation. There are plenty of developers who manages the distribution (packages, bug fixes, architecture...).
Debian overview of the packages : http://packages.qa.debian.org/s/scilab.html
Debian bug report for Scilab : http://bugs.debian.org/cgi-bin/pkgreport.cgi?src=scilab
SVN Debian packages sources http://svn.debian.org/viewsvn/debian-science/packages/scilab/trunk/
Compilation report on various arch https://buildd.debian.org/~luk/status/package.php?p=scilab
Usually, Ubuntu packages are those from Debian plus sometime a few modifications. Ubuntu Scilab packages are created from Debian's and sync regularly.
Home of the package https://launchpad.net/distros/ubuntu/+source/scilab/
Bug tracker ubuntu https://launchpad.net/distros/ubuntu/+source/scilab/+bugs
There is a package done by the Italian Slackware Community http://repository.slacky.eu/slackware-12.2/scientific/scilab/5.1/
There is a package done by Simon Lipp: http://aur.archlinux.org/packages.php?ID=19897
Scilab exists in Mandriva. Mandriva has joined the Scilab Consortium.
Scilab Toolbox ANN : http://cvs.mandriva.com/cgi-bin/viewvc.cgi/contrib-SPECS/scilab-toolbox-ANN/
Mandriva bug tracker : http://qa.mandriva.com/
Gentoo is also a non profit organization. The main principal of the distribution is "compile everything with optimization for your CPU". The dependency tool (emerge) is very efficient.
Markus Dittrich <firstname.lastname@example.org> is the gentoo maintainer for a few scientific packages including Scilab. He is doing a good working on the gentoo packaging of scilab (few patches). Scilab 4.1 & 4.0 are available.
Gentoo overview of the packages : http://www.gentoo-portage.com/sci-mathematics/scilab
CVS gentoo packages sources http://sources.gentoo.org/viewcvs.py/gentoo-x86/sci-mathematics/scilab/
The work on the Scilab 5.X package http://bugs.gentoo.org/237572
Fedora is the community/testing version of the well-known RedHat distribution.
Package database information https://admin.fedoraproject.org/pkgdb/acls/name/scilab
Compilation report http://koji.fedoraproject.org/koji/packageinfo?packageID=12847
Fedora bug report for Scilab : https://bugzilla.redhat.com/buglist.cgi?component=scilab&product=Fedora
FreeBSD is a free (as speech and beer) Unix. A package is call a "port". They packaged the latest version of Scilab (4.1).
FreeBSD patches for Scilab : http://www.freebsd.org/cgi/cvsweb.cgi/ports/math/scilab/ http://www.freshports.org/math/scilab/
Suse / OpenSuse
Scilab is available for both Suse & OpenSuse