Thanks...that write up gave me an idea. I searched the registry for "WAN Miniport (Network Monitor)" and found a section that listed 13 Miniport entries. The last entry was the one identified as "WAN Miniport (Network Monitor) #2" which I figured was an "extra device" created as a result of some screwy interaction between my particular system configuration and SP1. I exported the entry "just in case", deleted it, and rebooted. The exclamation mark is now gone and the system is working correctly.
Maybe this will help someone troubleshooting a similar problem...