Drag bookmarklet (below) to your browser's bookmarks bar.
Select All/Unselect All Bookmarklet for FacebookBookmarklet does not work in IE7 or less.
Go to Facebook, click on Events, then click on Create an Event.
Click on Select Guests button, and then click on Bookmarklet in your bookmark bar. Please note, if you close and re-open the Select Guests pop-up, you will have to click on the bookmarklet again to bring back the select/unselect functionality.
Update: This bookmarklet no longer works due to a change made by Facebook. However, all is not lost. You can instead use my new Select All / Unselect All extension for Chrome!