To me, whether or not you achieve the business case is the only true measurement of the effectiveness of a public relations campaign. Anything else smacks of insider navel gazing.
Now, marketers have spent a lot of time quantifying paid media for good reason. It's much easier to generate good data for a media buy or a direct mail campaign. It's easier to make a direct connection between one's actions and the movement of the needle. Certainly, one of driving forces behind the massive investment in online advertising over the past few years, has been the huge reams of data it generates.
However, as the online advertising machine bumps up against user generated communities, it becomes harder to draw a cause-effect relationship. As Roy says,
Food for thought on a Monday morning!
...for emerging media, you may have to be satisfied with qualitative measures of impact. At least for now. After all, if you have two readers of your blog, and those two readers have the first name of Steve (Balmer and Jobs), your blog may be far more influential than another blog on technology with thousands of readers.