Here is a good article from Bruce Schneier discussing why, even if a "law enforcement backdoor" could be made hacker proof, it will not solve the problem it is intended for.
A "backdoor" is a deliberately introduced security hole that, in this case, allows anyone with the relevant knowledge to eavesdrop on communication. This does more than introduce wiretapping capabilities into private communications. It increases the complexity of the system which decreases reliability. It makes patching bugs more difficult because the wiretap functions will have to be validated. Additionally, if anyone really thinks that a mandated backdoor will remain "law enforcement only" he needs to look up the Athens Affair.