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1. Many search engines treat subdomains as separate sites. This can mean you get extra positions in the results for some keywords. It can also help focus the various themes of your site semantically, and that can boost ranking for some searches.
2. A year back or more there was a lot of SEO that played off "wild card" subdomains. Anything you searched on seemed to have a subdomain with that word. Search engines did not like this and took steps to minimize its influence.
So when you say "major sections" make sure the sections really are major in size and relatively stand alone for content, with minimal but effective cross-linking. If that's your set-up, then you may get treated like about.com, for instance, who has used subdomains to good effect going way back.
But if you only have a few pages with minimal content, you may get treated like the wild-card folks did, and wake up to find everything has vanished from the SERPs.
Try the site search (above) or a search on Google with 'site:webmasterworld.com' in the search terms. There are many threads around here that deal with pieces of this topic.