Here is an article posted on this site today that explains how to do this for a "wrapper" around sp_send_dbmail:
http://www.sqlservercentral.com/articles/Security/68873/#
This is another article, this one wrapping CREATE LOGIN:
http://blogs.msdn.com/lcris/archive/2005/06/15/sql-server-2005-procedure-signing-demo.aspx
David Lathrop
DBA
WA Dept of Health