Was recently paid in cash by a Craigslist transaction (yeah!), and one of the hundreds I was given had these very funny tiny ink stamp marks on them. I assume they're from a bank, or a corporation, who counts the cash, then someone stamps it as being inspected. After a while, I imagine some bills may become littered with innocuous graffiti. I assume $20's are most prone to vandalism, as they'll be highly trafficked and most popular.
Update: The marks are called 'Chop Marks'
Here's an interesting backstory about where these marks come from and why they are there... Just Teejay (a commenter) pointed them out on his linked blog, and I'm happy to pass on the news! (Dead link, huh? The short story is that Foreign owners of American bills in those Foreign lands will mark them with small innocuous logos so they can be identified in case of theft, as no one has 'extra' American bills around and you can identify who has stolen your American bills - most often done in China & Japan.)