If you've used any on-line collaboration tools such as Live Meeting, WebEx, Acrobat 8's built-in Acrobat Connect (formerly known as Macromedia Breeze), or even the old WebDAV commenting system distributed with Acrobat 6 onwards, then you'll probably be interested in checking out Rosebud.
Rosebud is a PDF and web-based Java solution that allows for genuine real-time collaboration. As distinct from Acrobat, there's no waiting, at all.
Rosebud PLM, CEO, John Mohan gave me a demo of the software at the last AGI PDF Conference in Florida and I was pretty impressed. He suggests it's a perfect tool to sit alongside Skype.
OK, so you want to stamp your document. Maybe you need to give reviewers some advice about the document's status or sensitivity. This tip from author Ted Padova demonstrates how to add stamps with the Stamp Tool along with related comments.