it depends how your syslogd is configurated. look at /etc/syslof.conf.
Iptables is a kernel tools so it posts log under the kernel name and the "notice" level of emergency (or is it info? I'm not sure)
If these logs get full (mean that your hd is full too) the system will probably crash,in fact, but that's not suppose to happen.
But remember iptables log nothing 'til your use the -J LOG rule.