We don't have any plans to make the full historical index available for free via the public REST API. The focus of
search/tweets (and the streaming
statuses/filter endpoint) will continue to be real-time search.
The commercial REST API from Gnip currently offers a full-fidelity 30-day index, and it's possible that we'll expand that over time. Gnip also offers the asynchronous Historical PowerTrack commercial product, which supports sophisticated multi-predicate search over the entire historical corpus.
For scholars we have a separate program, including the Twitter Data Grants initiative, to make data available to researchers.