hi allelopath,
I'm sure there are more apps to do what you want, but the only one that comes to mind at the moment is KSim. It has a "led type" lights feature to monitor internet traffic. It is very similar to GKrellm, and can even use it's skins. Among it's features is the Net status plugin, that will monitor dial-up as well as ethernet. You can read more about it's features here: http://ksim.sourceforge.net/