Basically it allows you to ‘bookmark’ the real world by adding location bookmarks in a social network/bookmarking service. So when you find the best cup of coffee in the world you can tag it and share it. Obviously you can do it online, but the more sensible use of it is that you can do it from your phone and even upload images.
All so far so good. I can see lots of great uses for it, but it may also be prone to a few problems such as massive amounts of “cheap computers here!” type spam, vigilantism (“pedophile lives here”) and (though this is good or bad) general badmouthing (“the service is crap here, this waiter sucks, etc."). I’ve already noticed “Kingsleys Chicken” over there in Australia… sigh.
Oh yeah, the AJAX interface doesn’t show the contents of the balloons in Safari either… But that probably Apple’s fault.