Being at a Cable/Media giant now, as you might imagine, we discuss advertising a fair amount, and in particular, exploring the strengths, weaknesses, and, really, differences in how the advertisers, content creators, and distributors think about the eyeball value chain on the web vs broadcast media.
One of the obvious but interesting observations for web metrics is that the commonly used measure for audience is not actually people. That is to say, the "visitors" referred to by "unique visitors" isn't people at all, but devices. And even that's a bit of a misnomer, because its really, for PC users, a per computer per OS user account metric. Whether its a browser cookie, Flash local shared object, or Google Gears data store (the latter two don't get cleared when you delete browser history in your browser, btw) - nevertheless, they are all at the same level of "user" granularity. I'm going to suggest that OS user account is really a poor man's device and data virtualization technology, much in the same way that Multifinder was a poor man's multitasking technology back in the days of the original MacOS, and thus, we're talking about a device metric.
Unique visitors (UVs) really is a direct measure of how many devices connect to a given site. And it is correlated, of course, but not identical to the number of actual users visiting that property. Some sites you may use only at home or at work (one UV per user), while some may be used at work and home (two UVs per user), or, in cases where many users share the device (home computers, or set top boxes, for example) it may be one UV for many actual users.
Magazines will often refer to the "pass along" index of a magazine: that is, how many people might actually read it, but may not have purchased it (House&Garden magazine has a pass along readership of 14 or 15 people per sold copy, whilst National Geographic is around 5 or 6).
In this area, the Internet is surprisingly immature, given the promise (and increasing reality) of behavioural, demographic, and metric oriented targeting of the world's many-to-many publishing medium. This kind of thing becomes important not just for CPM advertising (that is, impression and brand based advertising), but even more so when considering the efficacy of CPA advertising (so called "Cost-Per-Action" advertising).
I mentioned magazines rather specifically, because it appears that Internet advertising growth is coming most directly from print and publishing, and not at all at the expense of broadcast (its shrunk nationally, but more than compensated in other channels). Perhaps we need to extend UV's to be UV/U's (Unique Visitors/Users), much as Nielsen's does for TV ratings/share to extend more actionable transparency to advertisers and targetting technologies?