CentOS - Apr 2010 - warning: only V3 signatures can be verified, skipping V4 signature

Hello,

Anytime when I issue rpm command, there will be a warning massage "warning:
only V3 signatures can be verified, skipping V4 signature".

For example,

[root at hadoop-datanode44 ~]# rpm -qa | grep -i post
postgresql-server-8.3.8-1PGDG.rhel4
warning: only V3 signatures can be verified, skipping V4 signature
postgresql-8.3.8-1PGDG.rhel4
postgresql-pgpool-II-2.3.1-1
postgresql-libs-8.3.8-1PGDG.rhel4
postgresql-devel-8.3.8-1PGDG.rhel4

What does this message mean? Thanks.
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