It looks like ping.pl script is not compatible with "ping" utility binary output because "icmp_seq" should be replaced by "icmp_req". The attached fix solved the problem for me.
--- /usr/share/cacti/site/scripts/ping.pl.orig 2010-07-10 00:33:46.000000000 +0200 +++ /usr/share/cacti/site/scripts/ping.pl 2010-12-02 16:39:22.748319129 +0100 @@ -4,7 +4,7 @@ $host = $ARGV[0]; $host =~ s/tcp:/$1/gis; -open(PROCESS, "ping -c 1 $host | grep icmp_seq | grep time |"); +open(PROCESS, "ping -c 1 $host | grep icmp_req | grep time |"); $ping = <PROCESS>; close(PROCESS); $ping =~ m/(.*time=)(.*) (ms|usec)/;

