Very good questions davide and answers eljefe3. Permit me to jump in, simply because youíve happened to start on about one of my favorites. I use sub-domains or canonicals as Iíve termed them, exclusively in my naming convention and for theme patterns. I have discovered a few things I am happy to share with you.
First, why are you using them? Knowing why you are using them leads to a very good explanation of how you are using them and that of course gives you the information you need to promote them. In other words are they duplicates in structure and appearance but with independent content, such as a vortal? Or, with very deep resources and content such in about.com? Is it a multi-faceted site, meaning strong separate themes that connect in a themed way, to garner the strength such organization can bring? Is it a simple one-theme topic with a few options to split it before bringing it back together?
Why and then how you connect your canonicals/sub-domains, what their appearance is and how independent they are from each other, will determine how far you can take each section and to what extremes you can market them independently and as a whole. About.com for example is throughout ODP simply because you cannot beat it for content balanced with resources. Itís the perfect model to go by in developing the highest form of themed canonical/sub-domain marketing.
With so many possible variables to consider itís hard to generalize. The most you could hope to achieve is a legitimate listing in ODP, Yahoo and the other directories. To achieve this, note the amount of variety and content you would need to provide and how you then go about linking it. The challenge is great and what a rush when you make it. Iíve done very well in getting a main page for the entire operation in the two main directories and the canonicals in all the others and then using a very themed linking process to bring them together.
I have a site right now that hasnít been submitted to the main directories and because of the linking and the strength of a sister site itís linked into, has only after 6 weeks online (no submissions), been picked up by Google and is getting a thousand unique and new visitors each day. I give lots of credit to the way I use of these canonicals. It isnít that I use them, itís how and why I use them, and to what level I take them, legitimately.
There are more opportunities to submit canonicals to a variety of vertical marketing sources and these can prove to be very lucrative in terms of traffic and strengthening in terms of link partnerships.
I hope this helped a bit.