In what file in Ubuntu 10.04 "Lucid Lynx" Server are boottime-activated iptables rules written by default?
Actually I look for it to set up Webmin to manage iptables rules.
From the command line you can follow the steps listed at https://help.ubuntu.com/community/IptablesHowTo (scroll down to the section titled "Configuration on startup") to save your iptables ruleset and have it load on boot.
If you used webmin to generate the rules I believe it saves a copy in /etc/webmin/firewall/iptables.save but I do not remember for sure.
By default Ubuntu Server haven't got any firewall configured, so no file is defined.
You can use Webmin without worries.