some good advice here. amazing how many webmasters you encounter that don’t realise that each link to their subdomains from a page on their root domain counts as an outbound link..
sub d’s are quite valuable /useful in many ways- and in my experience it is easy to spread good PR to each, if you link to each of them from each page of each subdomain at the start of the markup html markup (e.g. in a navbar- you can of course position it wherever you want on the page using css)….
as the prev poster said, don’t go overboard with the crosslinking – as long as you are sensible you won’t get nabbed/flagged for what might appear to be link-farming – and you can enjoy a more organised site, and tasty seo benefits from having more extra relevant keywords in the url to each subdomain page (assuming your subs are semantically justified with the content you would expect given the subdomain name).
just my .10c