Multi-tab browsing is now the norm, so you can’t assume the user is watching your app just because it’s running. Fortunately, the new “Page Visibility API”: http://code.google.com/chrome/whitepapers/pagevisibility.html lets your app discover if it’s visible or not. You could use the API to cut down on unnecessary network activity and computation.
@document.webkitHidden@ is a boolean value indicating if the current page is hidden (you can try it now in the console if you’re using a recent build of Chromium). @document.webkitVisibilityState@ will return a string indicating the current state, one of “visible”, “hidden”, and “prerendered”. And a new @webkitvisibilitychange@ event will fire when any of these changes, e.g. when the user opens you app’s tab, or moves away from it.
If you’re interested in giving this a whirl, check out visibility.js which adds a little bit of sugar on the API to make watching these interactions a bit more fun.