Just make sure its totally secure and you're actually in really good shape. If things get real "intimate" and she can't stop grabbing your hair, then let her go for it. You'll find that if its secure, you will feel her maybe touch the piece, but she won't think anything of it. Remember, WE know its there but she doesn't by default.

If she lost a tooth, how quick would she have a dentist replace it? If her breast fell off, how quick would she get an implant? If you lost your hair, how fast would you be ordering at TopLace? :-)

If she does figure it out (which will take a LONG time of messing your hair up if it's attached right), then you can joke and say "Got ya, didn't I? It's totally fake, pretty cool huh?" and have some fun with it. Like stated above, the right girl will NOT care one bit as long as you look good. And if it's just a one night stand, you wouldn't care anyway.