Here -> [http://trac.wpwp.org/wiki/Documentation/Playing/Spy_Visitors] you can read about the SPY visitor view ("The SPY visitor view is a cool way to watch what visitors are doing on your blog in very real time. It's inspired from the Digg spy view.").
Can this be done for piwik as well?
From what I can see this is already implemented in the LiveVisitors plugin (core distribution) and available via the Live! Widget.
As halfdan said, this has already been implemented. If you upgraded from an older version of Piwik, you'll have to activate the Live plugin and add the widget to your dashboard. (This is activated by default in newer releases for new installations.)
From which version does it belong to 'core'?
My mistake. The Live plugin is not activated by default. (It was the Goals plugin that was activated by default.)
Just go to your Settings | Plugins. Scroll down to Live, and click on Activate.
Live should be enabled by default (if it isnt it's a bug). Also the Spy widget should be in dashboard by default.