Both PEW and Gallup (others?) have examined public support for torture.
In a lengthy report on a variety of issues, PEW has these results:
The full discussion of these results is on page 25 of the PEW report (page 29 of the pdf file).
Gallup editor Darren Carlson discusses a November 2005 survey here (n=515). The question was, "Would you be willing -- or not willing -- to have the U.S. government torture suspected terrorists if they may know details about future terrorist attacks against the U.S.?" The results: 38% were willing; 56% were not willing; and 6% had no opinion.
Update: Here's an ABC News/Washington Post poll from May 2004 (n=1005), which found 35% of Americans thought "it's acceptable to torture people suspected of terrorism in some cases." The poll also covered specific techniques for "getting information from prisoners," such as sexually humiliating suspects (16% support) and not allowing a suspect to sleep (66% support).