This is where the distributed comes in. Micro-blogging is all about posting little notes, and subscribing to other people's little notes so that conversations take place. Since Identi.ca is open source, there could be any number of installations on any number of servers at any time. So, they implemented the OpenMicroBlogging protocol. This allows you to subscribe to a user even if you're not on the same server as them.
Where is Drupal in all this though? I want to be able to allow other to subscribe to my Drupal posts from Identi.ca, and subscribe to other people posts on my Drupal website. Think a Identi.ca implementation on top of Drupal. My Drupal installation becomes my Identi.ca profile.