Bill Ives has done his first podcast with the Otter Group's Learning2.0 podcast series. His topic is Blogs as Personal Knowledge Management, and he does a nice job of summarizing in six minutes what blogs are and how people use blogs as their "backup brain." Bill acknowledges Jim McGee as the inspiration for the term, but I see in Jim's note that there is a weblog called Backup Brain!
Essentially, this form of blogging lets me record notes on interesting websites, books, articles that touch upon topics of particular interest to me: knowledge management, theory of constraints, technology, personal effectiveness, etc. Assuming the blog has a decent search or archival tool, I can search for or otherwise find those old references. Bill also acknowledges that many people start blogging for other reasons and then discover this backup brain feature by chance.
There is an important aspect to personal knowledge management in considering what tool(s) I use in recording notes, whether they are for personal or public consumption. The tool has to suit my needs, and I might discover those needs as I try out the tool. If I like to just keep lists of links (bookmarks), then something like furl or del.icio.us makes more sense. If I want to keep my notes private, then there are simpler options, like text files or Outlook's Journal feature. (Jim McGee keeps a private blog running on his laptop for just this reason.) And of course, I need not use my blog as a backup brain - there are plenty of ways to use blogs.
Note: There are other bloggers who claim their archives are useless: evidence that they are writing in a different form than backup brain.