One device that controls every bit of security in your entire life? That sounds smart...About a year ago, I saw some mention of work being done on an "iKey" (the actual word iKey is trademarked by different companies) where the iPhone could disarm a house alarm, work as a remote, work as a RFID badge at a company, provide proximity detection so if someone walks off from their computer, it would lock their machine, etc.
Since that Slashdot article, I've not seen much since. I'd love the idea though, and it could be done in a decently secure manner, mainly by finding a way to key in a code offline, and pair the devices up similar to a Bluetooth pairing. The latest Bluetooth standard supports very low voltage/small battery uses, so even that might be usable. This way, once the devices are paired, it would just take an app launch to open something.