I only see two groups who derive any real value from pings, the Content Publishers (or 'Bloggers') and the Content Aggregators (Including Search Engines).
I believe that neither of these groups can derive significant value to running a ping service. A service takes money to operate. To spend money, you have to make it somehow, and you can't make money by redistributing packets over the world, and not charge anyone for it. If its a free service, your company still needs to derive value from it. There is value to users using your website. There is not any value in you telling everyone about their website for free.
For the Content Publishers, the value of pinging is that you make sure everyone you care about knows about your new content. But, if they really care, they will be crawling your RSS Feed anyways, because pings are not reliable. As a content Aggregator, you cannot just trust pings. If for whatever reason you miss a ping, regardless of it was your fault or the pingers fault, you just missed content, which is the Cardinal Sin of an Aggregator. People want their content, and any failure to reliably Aggregate content will drive people away from your service.
For the Content Aggregators, Pingging doesn't provide many advantages. Google already indexes my RSS Feed every 15 minutes. Bloglines tries to do it once an hour. Even if you could update your search index in under an hour, no one cares, yet. The speed that information arrives to most people hasn't become critical. If I read an article at 5:45 or 6:30, it doesn't matter much. A full 24 hours, yes, that can matter, but the times we are talking about are already sub-1-hour. I believe there isn't much value to getting the data a few minutes earlier.
For a Content Aggregators, aka the people who make any money off this, there is little value from Pinging, because you can't trust the pings, and because your infrastructure is already built to crawl every RSS Feed ever, every hour. But, these Aggregators are the only ones who derive enough value from pinging, that one of them might try running a ping service, except for one major problem.
The content publishers want lots of people to know about their Feeds. They don't want only Yahoo or just Ask Jeeves to know. They want all the people they care about. That means your ping service needs to send pings out to everyone, and guess what, everyone includes your direct competition. And important people don't like their company helping their competitors, for free.
This isn't a new type of problem. In the conventional world, this is where the government steps in. They build roads, goto war, and build other public works/utilities. Because while the individual value to the individual user or corporation is minimal, the combined value is potentially huge. The problem is there is no utility on the internet. No group to take responsibility to ensure some things *just work. *Of course, everyone knows that roads aren't free, we all pay taxes.
But wait, the internet is awesome you say! Its special and we can solve these problems!
Guess what, there's more!
Smart people already solved this problem. Its called reliable IP multicasting. It is a whole different subject, that of replacing the current ping system, but it could be done. There are much much much much much better systems and designs for a single entity to distribute reliably a message to hundreds or thousands of other nodes, without causing excessive load on the system. I am sure someone has a PHD thesis out there on it too.
In closing my point is that the economics of pure blog ping services will never work. There is not enough value to either of the major groups to support these services. This means in the long run, all of them will die. I believe there could be a technological solution, but it would be a massive departure from our current system, and would require actual brain power to make it work. It isn't impossible, but services that require brain power to work have a tendency to not succeed on the Internet. This means we will walk around in our crap with some groups trying to make ping services work, until they collapse and we can all wait for more groups try to reinvent them as Web 6.0 in a few years.