Way back in December 2005 I blogged on the quick uptake of Subversion on RubyForge. That trend has continued and today, 10 months later, Subversion and CVS are almost even:

 select p.plugin_name, count(gp.plugin_id) from groups g, group_plugin gp, plugins p
   where gp.plugin_id = p.plugin_id and p.plugin_name like 'scm%' and g.status = 'A'
   and g.group_id = gp.group_id group by p.plugin_name;
 plugin_name | count
-------------+-------
 scmcvs      |  1110
 scmsvn      |  1033

So, Subversion is only 70 projects or so behind CVS and gaining fast. I bet it'll overtake CVS by the end of the year. If you want to convert your repo from CVS to Svn, just file a support request.