cannot delete application on Lion
There are 2 issues:

First, you can't delete applications from Launchpad. You need to delete them from Applications. 

Second, you can't delete some system applications (that are essential part of OS X) from Applications by dragging them to Trash. You can only delete applications that you've added. (Actually you can delete system applications, but not by dragging to Trash; it requires some command line interface usage).