From the IRS website:

Beginning in 2008, small tax-exempt organizations that previously were not required to file returns may be required to file an annual electronic notice, Form 990-N, Electronic Notice (e-Postcard) for Tax-Exempt Organizations not Required To File Form 990 or 990-EZ. This filing requirement applies to tax periods beginning after December 31, 2006.

Small tax-exempt organizations, whose gross receipts are normally $25,000 or less, are not required to file Form 990, Return of Organization Exempt From Income Tax, or Form 990-EZ, Short Form Return of Organization Exempt from Income Tax. With the enactment of the Pension Protection Act of 2006 (PPA), these small tax-exempt organizations will now be required to file electronically Form 990-N, also known as the e-Postcard, with the IRS annually. Exceptions to this requirement include organizations that are included in a group return, private foundations required to file Form 990-PF, and section 509(a)(3) supporting organizations required to file Form 990 or Form 990-EZ.

NOTE:  Organizations that fail to file the e-Postcard annually for three consecutive years will have their tax-exempt status revoked.

Read more about the e-Postcard here.

Also check out the IRS FAQs for more details of the new requirement.