1) If you create a backdoor for the "good guys" to access something of value, then it's only a matter of time before it's used by some "bad guys". The time it will take for this to be compromised is inversely proportional to the value of the information. Considering how many corporate secrets are on Blackberries, I'd say it wouldn't take very long (although such a breach might not make the news, or even be discovered at all).
2) Anybody with any brains or resources is going to come up with his/her own way to encrypt communication rather than relying on a Blackberry / Skype / etc. One nice thing about modern cryptography is that you don't need to be an expert in it to use it effectively. To put it another way, the people you're hoping to catch here are the subset of criminals who know that they _should_ use encryption, but don't know that the product they're using has a built-in backdoor. Personally I think that's probably a small subset of criminals, but maybe that's just me being naive.
This is completely ignoring the surety that any such backdoor will be abused by the guvmint, of course, but when I start going down that road I sound like I should be wearing a tinfoil hat.
--Nate
