Apple has said that for legal reasons, it may be forced to keep the code permanently and will have to secure it permanently out of concern for future legal/court obligations specific to this case.
In support of this, someone forwarded me a very interesting article written by someone who creates forensic software for a living. The legal requirements surrounding the creation of a software tool for forensic purposes, which this proposed effort requested by the government might fall under, are nothing less than herculean in scope.
I'm sure defense counsel would want to be able to verify that it isn't modifying file access times, or deleting data, or planting data, or otherwise disturbing evidence when the update is put in.
http://www.zdziarski.com/blog/?p=5645