The plate comes in the box (or at least, it did with my wife's phone).
Microsoft is buying an entire generation of imaginative, sand-box gamers with this purchase. Seems like a good idea. No idea if $2.5bn is too much.
There are some apps out there already that can use existing API's to mess with the lock screen. I use "Lock Screen Changer".
Guardian of Light is extremely good, especially in co-op. I'd recommend it.
Thanks very much Paul, I'll be grabbing this and having a good old read. Have you considered a tip jar for those of us who think free is a little too cheap? :)