Then you need to rethink your build procedures, seriuosly. If you've bothered to do a Subversion checkout, building a tarball with "tar czf ---exclude=.svn" of the checked out copy is trivial. If you're SRPM's do not include a tarball for a significantly sized source, you're usually doing something wrong: building RPM's from SRPM's that do not contain source is highly discouraged, and should be only used for packages where licensing reasons prevent the inclusion of the actual source code.
There are source control systems that can authenticate and verify the upstream repository, but without this, you're completely reliable on the upsteram source repository's security and authenticity for rebuilding your RPM's. That's a fairly unfortunate practice.
