My observation is the fact that google does not rank us dot.tk domains as highly as other premium tlds like .com .internet .org etc.

Is correct? Does anybody have evidence in either case that google ranks us dot.tk domain names comparable to or less than premium tld's?

.tk is really a country code domain for Tokelau. I imagine these aren't rated highly in other nations. In my opinion they apply it localized search engine results - http://www.seroundtable.com/archives/021424.html