Attention all plugin authors:
If you were using register_activation_hook() to also handle updates from older versions of your plugins, you will not be able to do so any more in WP 3.1: 
The activation hook is now fired only when the user activates the plugin and not when an automatic plugin update occurs. This is consistent with how the deactivation hook works.
There is a proposal for a register_update_hook() instead: #14912
Also, the callbacks for de/activation can now accept a network activation flag:
There was some ancient code in WP::parse_request() that looked in $GLOBALS when setting up query vars.
This is no longer the case: 
As discussed in yesterday’s meetup, here’s a list of proposed changes to the Components in Trac:
1. Merge Optimization into Performance: they’re basically the same
3. Create Libraries component: tickets dealing with upgrading third party libraries, like jQuery or SimplePie, should go there.
4. Merge TinyMCE into Editor: there should be a single component that deals with the editor as a whole.
After this is done, I plan to document each component in the Codex, so that we have a clear description of what goes where.
Here’s a quick preview of the new menu management admin page (still alpha stage).
It highlights the dropdown section, which is the only unfamiliar element. All the rest are borrowed from the widget management screen.
Feedback on the UI is very welcome, either here, or on the dedicated ticket: #11817.