I can't solve this issue, but I ran into the same problem (with a CCBOX-field) and did some digging in the source code.
First of all: I'm not a PHP guru! I simply hope to get the ball rolling again on this one, since it's a bug that deserves (and needs) to be solved.
Here's what I came up with: it's all in the file cforms/cforms.php
It appears to me that the Initial State of a checkbox is being handled by a variable called $preChecked.
This variable never seems to get the value
checked="checked"
which is what we're all looking for.
I hope this will lead to a breakthrough in solving this issue.
Cheers,
Wim