Total size: 471 MIs this ok [y/N]: yDownloading Packages:warning: rpmts_HdrFromFdno: Header V3 DSA signature: NOKEY, key ID 897da07arhel-x86_64-server-5/gpgkey | 1.1 kB 00:00Public key for NetworkManager-0.7.0-13.el5.i386.rpm is not installed[root@rhel yum.repos.d]#
You can bypass the GPG check, if you want to take that risk. It'll probably resolve itself if you don't want to take that risk and just try again tomorrow.
yum --nogpgcheck update NetworkManager