Since these threads are so simillar I've merged them for you!
Anyway... to answer the questions...
plain edge means no detail, like serrations or hooks, it does not mean blunt.
but if you are buying a knife to go on your wall then this doesn't count...
Also I don't think that knife laws apply when it is in your own house...
UK knife laws state public place, meaning anywhere open to the public, (however this does include your property outside eg your car or garden), it doesn't include your mantle piece.
So far as getting an ornadental knife goes, you could probably do a lot better than a butterfly knife, there are some real feats of engineering when it comes to knives. like detailed blades and everything, these would make much better ornaments than a butterfly knife...
I can't say about US laws, but certainly in the UK it's illegal to carry a knife in public, (without good reason -e.g a chef can carry a knife roll).
it's illegal to carry a double eged knife. or a knife over a certtain length (2 1/2").
it's illegal to carry locking knives, (like butterly knife or other locking knifes, including folding knives with a pin or spring lock), it's illegal to carry automatic knives, (flick knives or switch blades)...
even though I've commented in this thread, (and hopefully helped a little).
I am in agreement with the other moderators,
all this talk of buying knives, or defending yourself with a knife, is a little out of place here...