Is the comcast device just a cable modem, or is it a gateway (modem+router)?
If it's a gateway then setting the router up as a DMZ device is the way to go. I'm testing DSL and they supplied me with a gateway, so I connected my router to one of the ports (gateway Ethernet port to WAN on router) and set it as a DMZ device. The gateway then assigned the router its own public IP and I had to reboot the router to renew its IP. After that it worked as before.
Note that if the gateway and router have the same base IP address range (192.168.1.xxx) you will have to change one of them. I had that issue initially, so I set the gateway to use the 192.168.2.xxx IP mask and resolved the issue.
Again, the above is only for an ISP supplied gateway. If it's just a cable modem you shouldn't need to do that.
I seem to recall some drama about Comcast blocking certain ports on their connection. That was a while ago, so I don't know if they're still doing that nonsense, but it's something else to consider... The fact that your computers work and your PS3 doesn't is what's confusing here.