Well i have to disagree here with most of the people saying you can not link from your mainpage, i do it all the time, and so what if google doesn't see your subdomain as a separate site?
Is there any proof that you you rank better if google does, and i highly doubt that google is seeing subdomains a sepperate sites because i have had domains banned with 20/30/60 subdomains and they where all gone, all the subdomains
So my conclusion is that G doesnt see subdomains as a different site and even if it does it doesn't matter for your rankings, just make sure that those sites you link to have good on page SEO and good content and you will be fine.
Ofcourse it is always good to link from your splogs the way i do it and are getting the best results is to have 3 to 5 splogs Freeservice>>>Freeservice>>>Freeservice>>>Freeserv ice>>>>Freeservice
And have all those freeservice blogs link to your blog on your (sub) domain, that is how i create one way backlinks and if you use my tactics on
splog building these backlinks will have a good weight overtime