The way WP.com supports domain names is messy.
WordPress.com automatically removes the “www” from all URLs. Mapping the “www” subdomain is not supported.via Map a Subdomain — Support — WordPress.com.
This is weird and messy. Either I can move my DNS / name servers to WP.com (which I am not comfortable doing - best practice is to keep your name servers separate from your hosting, so you can redirect if necessary).
Or, I can choose a subdomain.
So, this is now blog.bmann.ca. I will likely move bmann.ca to Octopress & an Amazon S3 bucket as well. Although I’m not sure that I need that top level domain doing anything, so for now, www and the root domain both redirect here.
Tumblr and Posterous both support root A records (which is messy because it ties into a single IP) as well as subdomain CNAMEs including www. If WP.com supported ‘www’, this would be a non-issue.
I guess this is another example of picking your use cases / target customers. Either host your full name servers with WP.com and they’ll handle everything, or be OK with the downsides of a non-www subdomain.