Wouldn't they have to prove that there was evidence pertaining specifically to the current case to get you for destroying evidence?
A judge isn't a robot. If you say "I hit an emergency button to reformat my hard drives as the cops were breaking down the door, because I was preparing to sell those drives on ebay and needed to clean them up", the judge is not obligated to respond "Shoot, I can't prove otherwise, you're free to go."
This seems like one of those laws that sounds good at first but makes a lot of normal behavior illegal, thus allowing the legal system to be able to pick and choose who gets punished.
The only marketable purpose for an instant erasure system like that is protection of information (personal or commercial). To build that, you'd make something that zeros all the files and deletes them, deletes and overwrites all the contacts, and such.
But the structures left after that don't look like a brand new device. They look like you had a bunch of things and then erased them. Cops won't know what you erased, but they'll know you handed them a phone that was recently erased.
Now maybe somebody will build an app tuned for obstruction of justice, so it tries to make the phone look brand new. And maybe you'll be very lucky and they'll get it working perfectly on your specific phone. But then you have to explain how you have a brand new phone that was actually purchased a year ago. And how it has no record of any of the calls that your phone carrier will have records of. The obvious conclusion is that you wiped it sometime after your last call and with special software that only appeals to people planning on hindering an investigation.
Is that enough to convict you for obstruction of justice? I have no idea; it probably depends on how much a prosecutor cares. But is it enough to convince cops you are vigorously hiding something? You bet.
No, there's no need for anything that complicated.
In principle, encrypting all data on a phone is really simple to implement. In practice, it's carefully thought out to avoid edge cases. The general idea is something like this:
when first activated, the
phone generates a random 256-bit AES key
phone uses this random AES key to encrypt
all data stored on the phone
phone retains this random key in a special
location, and encrypts this key by using
the user-provided PIN
It isn't necessary to erase an entire device. It's merely necessary to replace a 32-byte field (that contains the true AES key) with 32-bytes of random data.
From then on, it doesn't matter what the PIN is. Data on the phone is jibberish unless and until the proper 256-bit AES key is produced. That key no long exists, so from that point on the only way to recover the data is by brute forcing AES, by trying all possible 2^256 combinations.
They look like you had a bunch of things and then erased them
No, what remains is indistinguishable from the case where the correct PIN hasn't been provided. Having "things" on the phone is no evidence of guilt. There is absolutely no evidence that the phone was erased. All that is known is that the provided PIN isn't able to decrypt the data.
What we were discussing is abakker's proposal for something that "wipes the phone", and I think my comments are still relevant to that approach.
Of course, a suddenly unreadable phone is still suspicious, but if your plan were perfectly implemented, it might be impossible to prove obstruction of justice.