we used something similar in a bunch of early web applications we wrote, but to track unique user base, many many years ago. Page would pull in a single pixel image from our website embedded in the client site, so we could keep track of whether they were paying for correct user license. Permission was granted somewhere deep in T&Cs.
Now its pretty much used by everyone in products like Webtrends. Ah, I wish we'd had the foresight to make that product :-) Never mind