To add to this, all TAF's in the U.S. issued by the NWS are now 30 hr forecasts to conform to the worldwide TAF format.
http://www.nws.noaa.gov/om/notification/scn08-55_30hr_taf_test_sep.txt
Not all of the TAF's will be 30 hour forecasts. TAF's valid for 30 hours are only at high impact airports. The rest of the TAF's will still be valid for 24 hours. The 32 high impact airports are listed below.
Airports for 30 Hour TAF
1. KATL The William B. Hartsfield Atlanta International
2. KBDL Bradley International
3. KBOS General Edward Lawrence Logan International
4. KBWI Baltimore-Washington International
5. KCLE Cleveland Hopkins International
6. KCVG Covington/Cincinnati
7. KDEN Denver International
8. KDFW Dallas/Fort Worth International
9. KDTW Detroit Metropolitan Wayne County
10. KEWR Newark Liberty International
11. KIAD Washington Dulles International
12. KIAH Houston – George Bush Intercontinental
13. KIND Indianapolis International
14. KJFK John F. Kennedy International
15. KLAX Los Angeles International
16. KMKE General Mitchell International
17. KMSP Minneapolis-St Paul International/Wold-Chamberlain
18. KOAK Metropolitan Oakland International
19. KONT Ontario International
20. KORD Chicago-O’Hare International
21. KPHL Philadelphia International
22. KPIT Pittsburgh International
23. KSAN San Diego Int’l – Lindbergh Field
24. KSDF Louisville, Int’l Standiford Field
25. KSEA Seattle-Tacoma International
26. KSFO San Francisco International
27. KSLC Salt Lake City International
28. KSTL Lambert-St Lewis International
29. KSWF Stewart International
30. PANC Ted Stevens Anchorage International
31. PHNL Honolulu
32. PAFA Fairbanks
http://aviationweather.gov/notice/taf30.php