From the EAA: “Ultralight pilots who registered with one of the four FAA recognized ultralight organizations before September 1, 2004, but miss the January 31, 2007, deadline, or pilots who registered after September 1, 2004, still may use the aeronautical experience (flight hours) they obtained in ultralight vehicles to meet the experience requirements to obtain a sport pilot certificate.”
Two PDF files are available for download regarding this issue:
Clarifying Information - http://www.sportpilot.org//interest/ul_deadline.pdf