Apple's (AAPL -1.5%) sell-side fans are quick to attribute the weekend's lower-than-expected iPhone 5 sales on supply constraints. Piper's Gene Munster notes Apple's retail inventory is half what it was at the time of the 4S launch, and says his prediction of 8M sales was based on a belief Apple's figures would include all pre-orders (apparently not the case). The WSJ reports this morning that retail partners such as Best Buy and Target were given limited supplies at launch time.