Been there done that, though this doesn't appear to be a blog issue. You can control how long you remain logged into a DNN site by modifying the timeout settings in the web.config.

  <add key="PersistentCookieTimeout" value="0" />

If I'm writing a long blog I try to always copy the thing before I hit submit, doesn't matter what blogging platform I'm using, I've lost enough things due to issues like you just had that I am far more cautious now.

Steve,

I'm really sorry to hear about losing your post.  I've been there too and I know that it's really frustrating. 

I realize this won't bring back all your hard work, but before I started using Windows Live Writer for publishing to DNN, I used the Telerik radEditor and one of the changes I made for one of my clients who had this issue one too many times was to modify the ctlRad_Load event of the RadHtmlEditorProvider as follows:

The change required recompiling the provider, but this modification saved the contents of the text editor to the clipboard before attempting to post to the server.  If the post failed, at least the contents of the post could be pasted again from the clipboard.  I have never taken the time to see what would be required to sneak this same fix into the FCK Editor.  Maybe someone that knows the FCK editor better than me would know where you could insert JavaScript to copy to the clipboard before the contents are saved.  I took a quick look and I couldn't find a spot that worked the same way the RadHtmlEditorProvider fix worked. 

Of course, I recommend now that clients use an offline client such as Windows Live Writer or Word 2007 to manage their DNN content for this very reason.  It's a great feeling knowing that as you write your post you won't lose it because of a session timeout or because some other program needed to open a web page over top of your hard work.