Hello, Mark:
I replied to another post which seems to be the same.
I think there might be an issue with either a) the browser setting for the checkboxes (right click, properties, browser and check always), or b) the value that you are using in your condition since checkboxes can have TRUE/FALSE, or 1/2 as values and that may be throwing off your conditional formatting rule.
Anyway, please read the other post as well, and email me if you can't figure it out.
Cheers!