Here is a thread about it:
http://answers.yahoo.com/question/in...5094134AAzw31x
For me, it's an ethical issue. The postage on a letter pays for its passage through the system once, not twice.
And here's a link to the U.S. post office poster on the subject, warning that it's a federal crime to reuse stamps:
http://www.usps.com/cpim/ftp/posters/pos5.pdf