Here is a yum transaction that shows that the %post script of the upgrade of glibc failed.
 
[lrineau@chagall ~]$ sudo yum history info 3
[sudo] password for lrineau: 
Loaded plugins: langpacks, presto, refresh-packagekit
Transaction ID : 3
Begin time     : Tue Jun  7 13:39:44 2011
Begin rpmdb    : 1604:6c12876c27e1e4baa675b94e4086c0940cf42091
End time       :            13:42:51 2011 (187 seconds)
End rpmdb      : 1606:f1c2fbae33040b7e9a207b7bb97c0b807c8c9315
User           : Laurent Rineau <lrineau>
Return-Code    : Success
Command Line   : update
Transaction performed with:
    Installed     rpm-4.9.0-6.fc15.x86_64
    Installed     yum-3.2.29-5.fc15.noarch
    Installed     yum-presto-0.6.2-3.fc15.noarch
Packages Altered:
[...]
    Updated glibc-2.13.90-9.x86_64
    Update        2.14-2.x86_64
    Updated glibc-common-2.13.90-9.x86_64
    Update               2.14-2.x86_64
    Updated glibc-devel-2.13.90-9.x86_64
    Update              2.14-2.x86_64
    Updated glibc-headers-2.13.90-9.x86_64
    Update                2.14-2.x86_64
[...]
    Install kernel-2.6.38.7-30.fc15.x86_64
    Install kernel-devel-2.6.38.7-30.fc15.x86_64
    Updated kernel-headers-2.6.38.6-27.fc15.x86_64
    Update                 2.6.38.7-30.fc15.x86_64
[...]
Scriptlet output:
   1 /usr/sbin/glibc_post_upgrade: While trying to execute /sbin/service child exited with exit code 1
   2 warning: %post(glibc-2.14-2.x86_64) scriptlet failed, exit status 1

Should not that script call systemctl now?