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