While not a documented feature, you actually can enter an arbitrary RDL expression into a textbox in a Report Builder report. For example, you could add a textbox that shows the date the report was run by setting the value of the textbox to the following:

   =String.Format("Date: {0:MM/dd/yyyy}", DateTime.Today)

Another potential use of this undocumented feature is to display the values selected by the user for each report parameter. This is a helpful workaround for the fact that the auto-generated filter description does not insert the run-time values for each parameter. Here's an example of an RDL expression that would provide this info:

   =String.Format("Sales Year: {0}", Parameters!OrderYear.Value)

Note that you will have to guess the name of the report parameter generated by Report Builder. Typically it will be the name of the field used in the prompted filter condition, with any spaces removed.

You cannot enter an RDL expression directly into a detail or subtotal cell in a table or matrix.