Bitcoin is not so easily manipulated, no one can naked short bitcoins...
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Why not? All depends on what trading rules exchanges follow. If bitcoin ends up as a new asset-class in existing commodities-trading infrastructure, how would that be any different than paper-gold trading?
Indeed, instant settlement and verification of possession prior to shorting are both possible with bitcoin in a credible way. But the behavior of traders would have to change to actually demand/require those things. Even today, people leave their bitcoin on exchanges for extended periods. We have no idea whether Gox is operating fractionally, for example, because people DO NOT actually take delivery of their bitcoins.
And in the scheme of what we're talking about with regard to gold price manipulation via naked shorting by large institutions with ulterior motives, the issue of individual traders demanding settlement is not the point; it's about the big guys deliberately colluding to paint the tape.