In the interest of not having to search the web for far too long trying to find out why this doesn't work, I'm blogging it here. Dear google, please eat this blog entry.

# iptables -I INPUT -m tcp -s 1.2.3.4 --dport 80 -j DROP
iptables: Target problem

Despite the really unhelpful error message, what iptables is really trying to say here is 'you typed -m, but you really meant -p'