Why? Because they've only indexed my blog up till 26 May 2005! What's up with that?!
SearchEngineWatch has a good summary of opinions in the blogosphere. Put me down as 'Underwhelmed' please Danny (sub-category 'Peeved').
UPDATE: I've discovered the issue is that Google Blog Search indexes the RSS domain name, not the blog one. Because I have a Feedburner RSS feed, it's indexing feeds.feedburner.com/readwriteweb as the domain name for my blog. This is true of other Feedburner customers too - e.g. John Battelle's blog has the same issue. So I've contacted Google Blog Search about this.