http://blogs.zdnet.com/security/?p=1212

Basically it allows a website to download stuff to the default download location (on Windows, the desktop, on Mac, the download folder) and in the future, possibly run the file right after downloading it. Now, programs are bound to have their bugs, but Apple stated that it was merely an irritant, and not a serious security issue, therefore, as it is now, they're not planning on fixing it until a major upgrade of Safari.