I'm giving away SEO technique tips for free here....
I have not found any evidence AT ALL to indicate a sub-domain would suffer adersely just because it is a sub-domain.
In fact if you visit some of the larger sites that take advantage of using sub-domains, you will probably find the sub-domains are actually ranked higher!
Why?
Well, when you think about it, it's obvious.
Why would you need a sub-domain?
Yep, thats right, to target a specific service/subject/product.
Lets look a little closer..........
You have a site called furniture.com
Now lets create some sub-domains;
teak.furniture.com
marble.furniture.com
Irrespective of any SEO techinques you apply, provided you apply the same techniques throughout, when searching for 'teak furniture' the teak.furniture.com sub-domain will ALWAYS be better than furniture.com by virtue of the fact the 'keywords' are contained within the url.
Thats *How* a sub-domain can and often does work better than the main domain
If you use Alexa or Alexa Ranking as a guide to your sites popularity, here's an interesting and useless fact. Once Alexa has indexed your site, any sub-domains will automatically inherit the main domains popularity rank (for three months).