(edit)
Sadly, not with iTunes directly.
It can easily be done with QuickTime Player & AppleScript.
Open the Script Editor.
Paste the following in and save the script as *Speed up playback* to username\Library\iTunes\Scripts\ folder
tell application "iTunes"
pause
set my_track to location of current track
set my_seconds to player position
end tell
tell application "QuickTime Player"
open my_track
set my_movie to first movie
set ts to time scale of my_movie
set current time of my_movie to my_seconds * ts
set rate of my_movie to 1.5 -- starts playing
-- set this to the speed you want to playback at. 1.0 = normal
end tell
Select the book in iTunes then go to the AppleScript menu and select the new script you created.
(from ->
Mac OS X Hints - Listen to podcasts at 1.5x speed