My name on stackexchange is Bruno, however there are multiple Brunos on stackexchange. When I search for Bruno there are many results but none are me. Is the search delimited by reputation? So if my reputation were a million and I searched for Bruno I would be the first on the search results?
Are you referring to http://stackexchange.com/search?q=bruno? AFAIK, that's the only way to search all the sites simultaneously. It's just a Google search, so all the normal Google SEO stuff applies. If your name is in the URL of your profile, your profile has been linked and mentioned a lot, and you've made a lot of posts (which have links through your user card back to your profile), you'll rank highly there. It's roughly the same behavior as you would see at a generic Google search like http://www.google.com/search?q=site%3Astackexchange.com+user+bruno
I rarely, if ever, use the Google search tool. Instead, I use the syntax documented on the search page:
to search for posts by a specific user. A userID is the number in a profile URL: