Hoooray! You do get a bonus gift for doing so, but while it's pretty, it doesn't sound like much. You basically get decals for your PSP and they contain the many symbols of Kingdom Hearts. Certainly sounds nice, but would it be enough for some people? I don't know. I DID pre-order 358/2, but that's because I had a SLEW of gift cards and a $5.00 off coupon I got from Squaresoft.
Will it be the same for Birth By Sleep? Time will tell. Lol! It certainly looks beautiful though. I make it a point not to look too much because of spoilers, but from what I've seen, it wouldn't surprise me if it were considered among the highest points in the entire series. Not that there was ever much of a low point.