A number of prominent scientists have been notoriously secretive. This secrecy seems to have been inspired both by the fear that others would steal their ideas and that they themselves might be proved publicly and embarrassingly wrong. Bee has a clever post suggesting that they had the right idea.
Her point is that infant ideas may need some coddling and protection before they go out into the rigors of intellectual debate. Too much information can choke off creativity.
This principle is not new. A lot of brilliant ideas were had by people who had limited knowledge of the state of the current science. Feynman and James Watson remarked on it.
A closely related notion is a key principle of "brainstorming." No one is permitted to criticize the ideas created during the initial phase.
So go ahead and keep your big idea secret until you are ready to spill the beans. Of course you might get scooped by someone more forthcoming.
Clearly the rules need to be slightly different for data obtained with communal resources.