DSL is an "always on" interface, so while the IP assignment is dynamic, unless you're turning off your DSL modem often, you're probably stuck with the same IP most of the time anyway. While the DHCP is usually set up to refresh the IP's every 24 hours or so, the DSLAM/CMTS only has a limited number of IP's to pick from, so usually you get the same IP on a refresh. My personal opinion is that for a home or SOHO setup, using NAT via a router is probably about as much security as you need. Remember, if a hacker doesn't know you have anything worthwhile on your network, why would they bother hacking it in the first place?