[What are the SEO effects of placing search phrases in a subdomain rather than buying a .com? Are subdomains considered independent domains by Google just like any other .com? For example: Would honda99.com outrank honda99.domain.com, and why?]
Google indexes and ranks individual pages, not domains. creating a page on a sub-domain is not much different than creating a page on your primary domain. This is also not much different than creating the same page on a web 2.0 property, except that a web 2.0 may have a community and may provide links to your page. In those cases where you are given links and traffic from within the community, you are seeing a boost from that, but nothing from the domain has any impact except for keywords in URLs.
When you use a keyword domain you are adding your keyword to the URL of every page of the website. Each page of your website usually links to the homepage boosting the overall relevancy score of your homepage. In addition to this internal relevancy boost, you also get the added benefit of your keyword in anchortext for every backlink to your page that simply uses your URL for the anchor. These are the factors that help you rank better from the use of a keyword domain.
It’s not the domain name that is a ranking factor, it is the keyword in the URL and the keyword in the anchor. Using a keyword domain is just a great way to take advantage of those factors.