Starting in January 2011, the National Science Foundation will require the submission of a two page data management plan as a part of all proposals for NSF grant funding. Proposals that do not include the requisite DMP will be stopped from submission. The change upholds the existing guidelines advocating open data, "[NSF] expects PIs to share with other researchers, at no more than incremental cost and within a reasonable time, the data, samples, physical collections and other supporting materials created or gathered in the course of the work."
This is considered a first step in what will become a comprehensive approach to data policy. Guidance from the NSF is general, leaving the specifics to be determined by the "community of interest", essentially the discipline to which the proposed project is relevant.
NSF initially announced the change in May, 2010.
The NSF has updated their Grant Proposal Guide, Chapter II.C.2.j to include information about the data management plan requirements.
Some subject areas, such as engineering or mathematical and physical sciences, may have specific guidelines addressing unique data management issues for that respective community. Be sure to look at proposal details and main directorate or division website for additional guidelines. Existing disciplinary requirements are posted at the Data Sharing and Dissemination of Results, which also includes the general data policy.
NSF also provides an FAQ on data sharing which will answer some questions about the requirements and process.