Hi Adam I came across this in my logs today too and have some further input. The 404s occur if you close the browser while on a page with the editor in it (and only then). It also only happens with Internet Explorer and not with Firefox. I spent a small amount of time trying to track it down and confirmed that the request for null appears to be only issued when the browser is closed, and at no other time. Here are a couple of examples from my log files, the first two are from the GotDotNet version of the community Starter Kit which I added Cute to yesterday and the last one is from a test page with just the CuteEditor on it:
2006-02-24 00:48:07 W3SVC1 127.0.0.1 GET /GCNDN2/Discuss/Private/null - 80
2006-02-24 01:12:51 W3SVC1 127.0.0.1 GET /GCNDN2/.NET+Books/null - 80
2006-02-24 08:11:22 W3SVC1 127.0.0.1 GET /EditorTest/null - 80
Just to re-iterate, this is 100% repeatable here - load a page with the editor on it, leave a couple of minutes so that all the load requests are flushed to the log file, then close using the close gadget on the browser. The 404 null request will be the next line added to the log.
Addendum: Something just occured to me that I hadn't checked out previously so I've just tried it. If you navigate away from the page with the editor on you also get the null request. Does the editor do anything when the OnUnload event is fired in the browser?