Building from Subversion instead of tarball

[Kenneth Porter <shiva sewingwitch com>]

All the packages I've built to date use a tarball as the base source. A lot 
of open source projects are going to Subversion for version control and I'm 
wondering if anyone's created a spec file that can serve as an example for 
using a repository tag instead of a tarball as a source? Would one need to 
do much more than replace a "tar xfz" with a "svn co" in the %prep? 
Anything else I should worry about?