||I got closer. I set up a new image service of the same data, this time checking the WMS box. This time ArcGIS Explorer finds a WMS server at http://host119.yakama.com/ArcGIS/services/naip06-3b/ImageServer/WMSServer, finds the service naip06-3b, but then reports that it was unable to add the service.
Could it be that my server is know inside our NAT firewall as web-lamprey, while it is know as host119.yakama.com on the outside? I had an issue with this (http://forums.esri.com/Thread.asp?c=158&f=1696&t=293083&mc=8|) and although when editing on the capabilities tab, I can change the url to use host119 on the right, but I'm unable to change it on the left which is still using web-lamprey.