So, riddle me this batman... If you store the key on the device and read it automatically on boot, how's that protect you? Or are you saying that it's on an external device so I now have to keep the "key" around to boot my phone? One offers zero protection, the other consumers will hate.
See this is what usually happens...The consumer doesn't want two devices to manage, they want one. We implement strong encryption using long keys, then we store these keys someplace "on the device" and protect them with a 4 digit pin. Consumers demand it. So we've really reduced the protection level of all that nifty encryption to that of a 4 digit encryption key.
Sort of like what happened to WEP.... It used good encryption (in fact we STILL use the same encryption for the most part) it just bungled the key management side of things to make it useable by consumers. (OK, they did some other stuff wrong too, but the problem was key management..)
So, I'm not saying that having a "boot key" device, simiar to an RSA token isn't a bad idea, I'm saying that most users won't stand for having something separate from their phone that they need to power it on, nor will they suffer though entering sufficiently long and complex passwords.