However, if you are going to provide individual homepages for the users of your web site and you want their URLs be mycompanyurl.com/username, like with del.icio.us, then maybe the domain option would be a better choice, and probably not only for your blog but for the rest of your corporate info as well.
I'd do a bit of reading on this and see if you can find some consensus. I don't think you'll find a ton of SEO experts here (though I might be wrong). Lots of opinions to be had at http://www.google.com/search?q=seo+subdomains&btnG=Search
For the record, I think the /blog option is the way to go (and I've had SEO somewhere in my job description for about a decade).