If I'm understanding the question, no, you wouldn't want bridged mode on the Airport--that essentially just passes traffic from the modem straight through, which is almost certainly not what you want to do.
I've got nearly that exact setup, and I'm doing what donmei is suggesting you do--the Motorola modem handles PPP (in fact, I seem to remember that it came preconfigured when I got it, or maybe AT&T's setup tool did it automatically), and the Airport set to DHCP on the WAN port and as "share a public IP address." It talks to the modem, the modem connects, gives the Airport an IP address, and then the Airport routes traffic as necessary to the internal network.
Provides some measure of protection from outside attacks on your computers (since the Airport should only pass incoming connections through if you've specifically configured it to forward that port), easy to set up, works smoothly.