First, their anwser is BS. I asked them go set me up for multiple machines and they told me just to use my own NAT.
More importantly, Comcast's DNS servers keep going down. Sometimes they are up on the net and the actual DNS daemon crashes. Sometimes the actual box is off-line. Either way, I'm glad I'm not a Comcast customer anymore.
If you have another ISP in the area you may want to get a hold of them. Try telling them you are a Comcast customer but are having problems with their service. Ask if you can get one month free or some other special deal to test out their service.