I think it's time there was a better general understanding of the shaky foundations of the blogosphere. Kevin Burton has been getting more and more upset with blog search. While pings have the potential to make the web far more efficient, the truth of the matter is that like any other open 'standard', they are just another vehicle for systematic abuse by spammers. On top of this, there is a pretty hefty chunk of legitimate use which is simply poorly executed (incorrect dates, lack of synchronization with the feed, etc.).
When one considers the expectations for real time search in the blogosphere, periodic crawling (e.g. crawling every t minutes, which is what I believe both TailRank and Techmeme do - please correct me if this is wrong) is not a scalable solution. Leaving aside the issue of crawling 10s of millions of pages multiple times a day, it also fails to provide an adequate discovery mechanism for new blogs.
While memetrackers are crawling something like 10s-100s of thousands of blogs, I don't believe they could cope with 2 orders of magnitude of scale. So once one accepts the ping ecology, one has to deal with spam. And spam filtering algorithms have as a very clear parameter the time taken to make the decision. If there is a need to make a quick decision - to ensure some sort of real time system - then that limits the evidence one can use to make that decision.
What I'm getting at here, possibly in a round about manner, is that things are a mess. One can have high quality - as the memetrackers give us - but limited coverage and discoverability; or one can have near real-time and better discovery (coverage) with the danger of spam. Getting high coverage, quick mean time to index and no spam is hard - perhaps the real problem is that we have transferred main stream web expectations (where sand boxing is appropriate due to a different view of real time) to the blogosphere.