Ask HN: Building page rank by blogging, do subdomains work?

I'm working building up the page rank for our new startup, Nyaruka (http://www.nyaruka.com). Now our particular vertical, SMS data collections apps for the developing world, doesn't necessarily have a huge audience, but our technical work using Django etc.. very well might.

So I've been starting to blog little technical tidbits here and there as we run across them, things that might be genuinely useful to others. I also have some other ideas for entries that would probably attract some clicks and links.

My question, am I wasting my time by having my posts hosted by Posterous on the subdomain (http://blog.nyaruka.com)? IE, how much page rank 'goodness' is transferred via subdomains? Should I instead just blog on our main site? What are the pros and cons there?

Thanks.