Does false positives appears so frequently?

Not so few cases then a malware sample is recognized as a Safe file (about the one in every 30 downloaded files)

> By the way, it is not good enough just to ask the user to "Trust" the individual file each time, because chances are every time a file is downloaded it will have a different name, so the "trust" list could end up with a huge number of files (unnecessarily).

a few piece of info:

Security data is binding with file checksum (for example you can see it in wikipedia as md5 ,sha-256 and other hash algorithms). checksum is changing if only file content was changed (filenames and extentions may be anything you want - it will not change the checksum).

(see video http://community.norton.com/t5/Norton-Protection-Blog/New-Feature-for-Norton-Internet-Security-2010-... )