The IRS has published the first Automatic Revocation of Exemption List. It indicates approximately 275,000 nonprofits whose tax-exempt status has been revoked because they failed to file a Form 990, 990-N, 990-EZ, or 990-PF for three consecutive years; the list is available for downloading in both Excel and PDF form. Here's the PDF of the New York State list.
As GuideStar notes, if an organization appears on the list, it must now must file a federal income tax return and pay federal income taxes. If it previously could accept tax-deductible contributions, it no longer can do so. "If your organization is not on the list, technically you don't have to do anything. This a good time, however, to reach out to your donors, funders, and other constituents and assure them that you are in good standing with the IRS. You may also wish to suggest that your supporters consult the list to ensure that the other nonprofits they give to are still exempt."