Just because the machine sees the NIC doesn't mean it automagically knows how to talk to the cable modem its connected to. It almost definately runs on one of 2 different protocols, the first is easy to get working, the other... a little harder. Let's hope its dhcp, try this as root from the command line:
If that doesn't work... i.e. it sits there like a dumb rabbit for 1 minute, then the cable modem runs off of pppoe... probably.
You have pppoe software on the redhat box, although I highly doubt that it will work as the roaring pengiun that shipped with RH is known to be a bit buggy. If dhcpcd doesn't work, check through the literature that came with your ISP (not the modem itself), and figure out if they use pppoe, or in the long form point-to-point protocol over ethernet. If that doesn't answer the question, call the ISP. Setting up the roaring pengiun would be a headache if its something bizarre.