Not a lot, but a couple every day on the server where I first saw this:
# grep -c 'getservbyname failed for tcp' /var/log/httpd/error_log*
/var/log/httpd/error_log:13
/var/log/httpd/error_log.1:4
/var/log/httpd/error_log.2:6
/var/log/httpd/error_log.3:7
/var/log/httpd/error_log.4:0
Grepping for the other message shows the same counts.
I see this behavior on at least four different servers, all of them CentOS 4 (none of our CentOS 5 servers show this), Plesk 9.5.3 (but also from when they were still on 9.5.2), both 32 and 64 bit. None of them have high counts, just a handful per log file.