When should Flask.g be used?

Advanced Flask Patterns, as linked by Markus, explains some of the changes to g in 0.10:

• g now lives in the application context.
• Every request pushes a new application context, wiping the old one, so g can still be used to set flags per-request without change to code.
• The application context is popped after teardown_request is called. (Armin’s presentation explains this is because things like creating DB connections are tasks which setup the environment for the request, and should not be handled inside before_request and after_request)