Add watermark to PDF

[John Almberg]

I've just spent a couple hours googling for an answer to this question 
without success... This is probably a bit off topic, but this list seems 
to be able to come up with answers to questions that stump other lists, 
so...

I would like to add a customized footer (a stamp or watermark) to an 
existing PDF, like the guys at Pragmatic Programmers do with their PDFs.

So basically this script would have to read in the PDF and (ideally) a 
plain text file, and output a PDF with the plain text merged into the 
PDF as a footer.

Anyone know of an existing utility that might do something like this?

Probably not, so anyone know of a PDF library that I could use to roll 
my own? I'm competent in Perl, PHP, and Ruby, and at this point, would 
be willing to learn Sanskrit if it could solve this problem for me. 
Unfortunatley, I've never done any PDF programming, so not quite sure 
where to start.

Any ideas, much appreciated.

-- John