Sellouts of the iPad at Apple Stores may be a good sign for investors in some respects, but they are also reasons to check optimism, says Piper Jaffray's Gene Munster. The analyst notes that after getting in contact with 50 US Apple outlets, 74 percent report being sold out of all iPad models, while 26 percent have some Wi-Fi models in stock. None have the iPad 3G. The numbers back similar ones from RBC's Mike Abramsky.