I figured out what this was by checking your website. I thought you might enjoy these photos from my back yard in a northwestern suburb of Chicago. I have never seen anything like this species before and almost crawled out of my skin when I saw them crawling all over a dead elm in my yard. The tree died after being infested with wood boring bees a couple of years ago.
Thanks for the info!
There are probably still wood boring grubs in your tree which is why the Ichneumons are laying eggs.
Ed. Note: (09/17/2005) We now believe this to be Megarhyssa macrurus which can be located on this site.