Is there a standard way to read / write face-tags from / to photo metadata?
Can I use it to import face information from popular photo organizers?
Yes.
The metadata working group (for example, Adobe, Microsoft, Sony, Nokia, etc.) has developed a standard XMP extension for this, see the image area metadata section in section 5.9 in the Metadata Working Group Specification .
Another XMP extension was made by Microsoft for the Windows Live Photo Gallery and is supported by the .NET API. See Overview of User Tags .
It looks like Google Picasa 3.9 reads tags from both sides of XMP, but only writes them to MWG.
More details:
What is the standard for storing tags and headings for faces?
Picasa and Windows Live Photo Album | Jeff Coop's Blog
Picasa 3.9 errors and questions (uploading photo orders, nonG + links and signing an account, sore name tags in the photo
Reading People Tags inserted into Windows Live Photo Gallery