I know that the real issue here is that most Flickr users have
absolutely no idea how to either find or change their licenses,
however two pragmatic technical pointers:
1. The flickrapi gives you back the licence based on a simple query
using the photo_id. I recently used this to double check licences
using a custom Python script during a mass upload. For Commons, it
would be not too hard to build friendly web-based simple tools to help
users check licences and even mass change their own licences in their
Flickr albums to be "Commons compatible" with a simple couple of
button presses (using oauth). The well designed Upload Wizard can do
the mass upload of their chosen albums to Commons.
2. I use a bookmarklet to quickly find images that can be used on Commons:
Here is an example search for "cookies" using the bookmarklet:
Personal and confidential, please do not circulate or re-quote.