backends/yum/yumBackend.py assumes that .discinfo is four lines long and contains a disc number.  The disc number is gone in F15, so as a result the backend will throw out valid media because it doesn't understand the new format.  See:  http://git.fedorahosted.org/git/?p=lorax.git;a=blob;f=src/pylorax/discinfo.py;h=24f435f4d8a8accf4804a21381c5e35b484a651c;hb=HEAD

I'd think the following patch would fix it up, though I have not tested it to be sure.

diff -ruN PackageKit-0.6.14/backends/yum/yumBackend.py PackageKit-0.6.14.new/backends/yum/yumBackend.py
--- PackageKit-0.6.14/backends/yum/yumBackend.py        2011-06-01 14:43:00.000000000 -0400
+++ PackageKit-0.6.14.new/backends/yum/yumBackend.py    2011-06-01 14:42:54.000000000 -0400
@@ -3488,7 +3488,7 @@
             else:
                 raise PkError(ERROR_INTERNAL_ERROR, _format_str(traceback.format_exc()))
 
-    def _media_find_root(self, media_id, disc_number=-1):
+    def _media_find_root(self, media_id):
         """ returns the root "/media/Fedora Extras" or None """
 
         # search all the disks
@@ -3509,7 +3509,7 @@
             f.close()
 
             # not enough lines to be a valid .discinfo
-            if len(lines) < 4:
+            if len(lines) < 3:
                 continue
 
             # check this is the right disk
@@ -3517,15 +3517,6 @@
             if cmp(media_id_tmp, media_id) != 0:
                 continue
 
-            # disc number can be random things like 'ALL'
-            if disc_number != -1:
-                disc_number_tmp = 1
-                try:
-                    disc_number_tmp = int(lines[3].strip())
-                except ValueError, e:
-                    pass
-                if disc_number_tmp != disc_number:
-                    continue
             return root
 
         # nothing remaining