Locking a computer into running signed code is just like programming Firefox to refuse to access https sites without properly signed certificates. There's nothi

Locking a computer into running signed code is just like programming Firefox to refuse to access https sites without properly signed certificates. There's nothing about it that's incompatible with a fully free, GPL software stack. In the case of Firefox, you're trusting the Certificate Authorities. In the case of the iPhone, you're trusting Apple. You could argue that Firefox leaves open vulnerabilities to things like Superfish (that Windows root CA didn't actually affect Firefox in most cases since Firefox keeps its own list), but you can take the signing concept as low-level as you like, including requiring a signed OS with a hard-coded, single software delivery mechanism which could still be fully GPL.