>Don't you need some iptable rules to take care of Network Address Translation, firewall, and inbound connections such as SSH etc.
Nope.
>Any reason you didn't go for Smoothwall, Redwall, etc ?
Not needed.
The ADSL PCI card is actually a router on a card, so it does NAT, DMZ's etc, all available via a browser.
You "can" add firewall stuff if you need it, but generally the NAT on the card will handle it.
(I'm sure "I" will add some eventually, but generally it'll work securely as-is in most instances)
The beauty of this solution is that the card presents as a standard network interface, so there are no flakey ADSL drivers to worry about.
Once you bridge your WiFi nic, Local nic and ADSL card, essentially you have one network which is all "inside" the ADSL router's NAT, *and* the entire network (wired and wireless) feeds off the DHCP server that sits on the ADSL card.

If I stuck a dedicated router distro on there, I wouldn't be able to use my Ubuntu desktop ...

It's now 9pm, probably peak time .. I have 10 active VPN connections running and 10 remote gkrellm sessions running over the ADSL.
This is a 30 packet ping against linux.co.uk;
--- linux.co.uk ping statistics ---
30 packets transmitted, 30 received, 0% packet loss, time 29042ms
rtt min/avg/max/mdev = 21.847/22.245/22.829/0.321 ms
Not only is it quick, but note the consistency!