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: