[pdftex] pdftex deterministic?

[Geoffrey Alan Washburn]

	I could swear I had read something about this in the past, but I 
couldn't remember the correct keywords to find anything via search.  In 
any event, I recently wanted to make some changes to a the source of a 
document and to make sure that these changes did not actually affect the 
document I tried diffing the before and after PDFs.  Unfortunately, 
after some further experimentation it does not seem that even repeated 
runs of the same document produce identical output.  Is there any way I 
can modify my documents or the parameters to pdftex to produce identical 
output on identical inputs?  I realize this very well may not be 
possible, and if so, what alternatives do people use in practice?  Thanks!