Will large Bitcoin holders be a threat in the future? There are most likely individuals or groups who hold (and hoard?) large sums of Bitcoin because of various reasons. I believe these people are the early adopters or miners of Bitcoin. Basically, we pretty much do not know anything about these people. Will they be a risk in the future?
Large Bitcoin holdings are static assets.  The price only climbs if there are active trades.  It would be in the interest of large holders to promote the use, trade and circulation of the currency.

If a large holder were to dump all their coins on the market all at once would cause the price to drop, but that would not be in their own interest, as they would lose their investment.  Then again, they can only get a return on their investment if they sell it.  It would be sensible that if a large investor wanted to liquidate their holdings, they would trickle them out slowly to keep the price high.  

Also, the total amount of money wrapped up in Bitcoins and the risk involved is not enough to attract really serious investors.  There are some large investors, such as the Winklevii, but they are attracting attention because they are the exception, not the rule.