I first spotted this a couple days ago at Gary Price's SEW Blog entry named Google Quietly Beta Testing Desktop AdWords Editor.
AdWords Editor is Google's free, downloadable account management application for your computer. Now you can download your AdWords account to your computer, make your changes, then upload your revised campaigns.
You can download it at http://services.google.com/adwordseditor/
I was waiting for a forum thread on the topic and I have found one at WebmasterWorld today. There AdWordsRep provides more details;
There is a huge faq at http://www.google.com/support/adwordseditor?fulldump=1 and if you like to apply to use the editor (Windows only) submit your request here.
Forum discussion at WebmasterWorld.