The feature requires the searcher to have a Google account, and all +1's appear in the searcher's Google profile. For example, mine is at profiles.google.com/u/0/john.swindells. You do have the option to show or hide the plusones tab on your profile, and it's hidden by default; it looks like Google has learnt some privacy lessons from its Buzz debacle.
To start using this feature before it is fully rolled out, you need to join the plus-one experiment at the Google Experiments page.
Eventually Google will be allowing website owners to include this button on their own pages, in much the same way as the "Facebook Like" and the "Twitter Retweet".
Overall, is this really going to make the Google experience more social - or will it be just another quality signal for Google's own search results?
To find out more, see the Google +1 Button page.