The DoS won't work on most of the specific example services mentioned in the post (Blogspot, GitHub, etc.), at least not if the user is using a modern browser, because most of the big names have such cookies blacklisted by browsers.
The mechanism is the Public Suffix List, which was originally created because there needed to be a list to keep track of which TLDs used public second-level domains and only allowed registrations in the third level. For example, while foo.example.com and bar.example.com are both owned by example.com, foo.co.uk and bar.co.uk are two different domains, since co.uk is part of the UK domain hierarchy (along with ac.uk and so on) and registrations happen at the third level. Therefore it would be undesirable if foo.co.uk could set cookies for the entire .co.uk, as in the UK ccTLD world that's equivalent to setting a cookie for all of .com.
So there's a big list (initiated by Mozilla) specifying that .com is a public suffix, .co.uk is a public suffix, etc., and wildcard cookies on public suffixes are refused. This has been adapted, as a huge hack, to big sites that have user-registerable subdomains. So now .blogspot.com is also treated as a public suffix, since anyone can "register" a foo.blogspot.com under it.
However new entries are added on a fairly ad-hoc basis, so a site that allows user subdomains that can run JS is vulnerable by default unless they explicitly get themselves added. I notice Dropbox isn't there, for one.
The list: http://publicsuffix.org/