Hi Mark,
ok, this is a guess mind, but I suspect you're embedding a NetWebForm inside a NetWebForm - probably by using the xHtml tab, and setting it to be "after <form>" instead of "before <form>". This results in the "Save" button submitting the wrong "form" - which means the checkbox value is not being sent to the server at all.
As an aside, a good thing to know, is that empty checkboxes are not included in the POST from the browser to the server, only "true" checkboxes are sent as part of the POST statement. This is probably hampering your debugging effort.
cheers
Bruce