Other Palm devices received this update back in July, this update brings no new applications, however there is greatly improved security and a few new features.
New features and improvements include enhanced support for the webOS PDK (Plug-in Development Kit), it also addresses a podcast downloading issue and gives you a better overall web experience.
If you have not yet received the update, it should arrive soon, if you are inpatient you can manually check for update by following these instructions.
Have you installed webOS 1.4.5 on you Palm Pre Plus? If so, are you experiencing any problems?