Comment 10 for bug 1003466

Looking further it appears that the get_addons() call in aptcache.py is mostly responsible.
Removing it:
=== modified file 'softwarecenter/db/pkginfo_impl/aptcache.py'
--- softwarecenter/db/pkginfo_impl/aptcache.py 2012-04-18 08:44:00 +0000
+++ softwarecenter/db/pkginfo_impl/aptcache.py 2012-05-31 12:57:32 +0000
@@ -665,6 +665,7 @@
             :return: a tuple of pkgnames (recommends, suggests)
         """
         logging.debug("get_addons for '%s'" % pkgname)
+ return

         def _addons_filter(addon):
             """ helper for get_addons that filters out unneeded ones """

and that "fixes" the memleak.