as always, if you are not sure about any of this stuff then make sure you have backups in place..
1] Download the 3.0 Raw upgrade package and save it somewhere..
url is
http://www.apple.com/downloads/macosx/apple/application_updates/digitalcameraraw compatibilityupdate30.html
2] Delete the current 3.1 Raw upgrade (the 3.0 cannot be installed until this is done) as follows
start a terminal session, at the prompt enter
*sudo rm -rf /System/Library/CoreServices/RawCamera.bundle*
enter your password when prompted
3] Install the 3.0 Raw upgrade that you have already downloaded, just click on the disk image and inside it has an installer.
4] go in to Aperture, choose Photo->Reprocess master for any images that were imported since the 3.1 raw upgrade, these should reprocess fine and revert to the original correct settings - haze / darkness removed.
hope this helps someone, I was worried that my images that had already been imported and then deleted from the memory card and then damaged by the upgrade would have been lost permanently, but it is a testament to the design of Apple applications that these changes can be reversed like this (just a shame the QA process didn't catch it though!)