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Developer Days Online 
The Wine – New Events
2009/2010 comparison
Revit 2009
Revit 2010
Here are the lists of event classes available in 2009 and 2010.  In 2009, Events are limited to basic document operations. Doc open closed saved add items to toolbars.

As you can see, there will be much more for 2010.  Views (when opened and exported) and document printing and pre and post events introduced. We know when something has happened and when something is about to happen. Many events are cancellable, for ex- if you want to check before printing, or saving if they follow standard and abort the operation. Post events help in creating log files or book keeping about operations.

Note:   Element-level events were out of scope for this release and (delete, create, modify) are not implemented.