It's the codered bug - here's how it works: the worm propagates by picking random IP addresses to be it's next victims. Most picks are on the local subnet, with some further away. The lights on your modem are reflecting all the ARP (addr resolution protocol) overhead associated with having lots o' traffic to random (i.e., mostly vacant) IP addresses. I'm watching the external interface on my firewall box with tcpdump right now-it looks something like this:

00:47:33.880536 arp who-has cx980340-a.dt1.sdca.home.com tell 24.13.5.1
00:47:33.914878 arp who-has cx701860-b.dt1.sdca.home.com tell 24.21.12.129
00:47:34.052775 arp who-has cx384380-a.dt1.sdca.home.com tell 24.20.246.1
00:47:34.090860 arp who-has cx943276-b.dt1.sdca.home.com tell 24.13.17.1
00:47:34.364243 arp who-has cx917811-a.dt1.sdca.home.com tell 24.20.246.1
00:47:34.392551 arp who-has cx953093-a.dt1.sdca.home.com tell 24.21.12.129
00:47:34.438820 arp who-has cx173469-a.dt1.sdca.home.com tell 24.20.246.1
00:47:34.604111 arp who-has cx248728-a.dt1.sdca.home.com tell 24.21.12.129

Cheers,

-Deke