In Moodle 1.7 if someone is editing a wiki page when you go into edit it, you get an error message like this:

This page is being edited by Annelie Chapman. They began editing at Friday, 5 January 2007, 12:29 PM and still have the window open as of Friday, 5 January 2007, 12:29 PM.

You need to wait for them to finish before you can edit this page.
You can override this user’s lock, but doing so may cause them to lose their changes! Please take care.

Note, this is the behavior of the default wiki, based on erfurt wiki. Not sure how the other wikis Moodle has will behave.