I was excited when I saw the title of the article, I thought perhaps SQL Server was keeping it's own data on when a stored procedure was last invoked, buried deep in the system area. In my case this wouldn't be easy to implement as we have several databases at my company which may use each other's stored procedures, so running one application won't do it.

What I have done in the past is a bit tedious, involving the source code for all the applications we have and all the reports. My stored procedure naming conventions make it easy to identify these objects, so I just have a "spider" program scanning everything and spitting out the name of the stored procedures. Then, everything that is not in the result list gets renamed temporarily with a chosen prefix until it is determined that indeed, it is no longer being used. Then it is deleted.

Your solution would work well for small applications but in a highly complex database environment it could be tricky.