Since I can't use my keyword in my domain name, it's taken already, can I use it as a subdomain and optimize some internal page. For example:
www.domainname.com/keyword
If I use my keyword as a subdomain whether this will improve my ranking?
Doesn't really matter where/how you use it in the domain.
I do a ton of seo business work and have never been effected by the google slaps or anything else because I stick to one core concept:
All search engines are in the information business and operate on 3 major elements:
Keywords a site is catalogged within the search engine's db system.
Onpage content relevancy to those keywords.
Traffic searching for those keywords and finding that relevant content.
It doesn't matter what you call the site/domain. All of the wild names of the biggest commercial sites prove that hands down. It is all about relevant connections of keywords and content onpage using the keyword as the subject, not the literal words needing to be mentioned like in old keyword stuffing days.
Examples: Google, ebay, twitter, digg, amazon, as well as countless others.
Make the name sake of your site something people will remember and want to visit again. Let the content, keywords and meta tags do the work for the traffic connecting with each other, as that is their purpose.
As for my personal opinion, I still use keyword meta tags, always have and always will. Even with google's own mention of using them more for adwords, it is still the backbone of the system for them. I am even willing to bet that they are claiming the lowering interest in the meta because they want to push adwords supporting sites higher in their search engine system, as well as to have more people use adwords in thinking that it has something to do with it other than extra marketing income for google...which it clearly doesn't.
Use your keyword meta tags, research the traffic to your keywords, updating them often and create keyword relevant copy onpage that supports your target keywords correctly.