A while ago I spaced out and ejected the disk image while the Text Edit doc was still open.
I think what might have happened is that you made some changes, however innocuous, to your open document before ejecting the disk image, and these changes were recorded by TextEdit's autosave function. Such autosaved versions are normally scrubbed when you quit the app (if you haven't manually saved in the interim, you'll get the "save changes?" dialog); but since you ejected the disk image without having quit the app, TextEdit begins subsequent launches with a quest to open the document in the changed state it was in when it was so rudely closed.
The autosaved version appears, in Snow Leopard, to be kept in
~/Library/Autosave Information. (There should be a .plist and one or more documents; the .plist maintains the location of the original files for which the autosaved versions were recorded.) My surmise is that you can put an end to the anomalous behavior by trashing the autosaved version, or perhaps by editing or trashing the .plist, but I make no guarantees!
