It looks like this:
The picture above is a map of the blogosphere from Matthew Hurst's fascinating Data Mining blog. Here's Matthew's description of the above image:
The dark edges show the reciprocal links (where A has cited B and B has cited A), the lighter edges indicate a-reciprocal links. The larger, denser area of the graph is that part of the blogosphere generally characterised by socio-political discussion (the periphery contains some topical groupings). Above and to the left is that area of the blogosphere concerned with technical discussion and gadgetry.
This kind of information is very useful for planning word-of-mouth marketing campaigns - as Matthew explains in one of his posts, the blogosphere is characterised by a number of very popular blogs which are linked to by hundreds or thousands of others. So if you want to reach a large part of the blog-reading public, you don't have to advertise on every blog; just find the ones which are linked to a lot - ideally from blogs which are in your demographic.
(Thanks to Mrs Thomas)