I agree with Simone, with the name
bingsandbox.com it seems very likely that the site will become a development environment without actually having any effect on their actual search results. Whether this be for the public or whether it be for people like us too test new features that they need testing and feedback, who knows. There doesn't seem to be any answer or news from Bing as to what this domain is for, and at present is just a mirror image of their current site.
What is interesting is that the domain itself has been registered in the UK and is being hosted in the UK, not USA. Also another interesting aspect is that Bing on
bing.com are currently using their own DNS servers and hosting, however pinging
bingsandbox.com reveals the following:
Pinging a134.1m.akamai.net [220.127.116.11] with 32 bytes of data
a134.1m.akamai.net is run by a company called Akamai Technologies who are a London based company that seem to specialize in content delivery networks, whether it be just HTML, CSS, JS or whether it be as complex as complete applications / systems. So my guess is as good as anyone's. Until they release a post explaining what it is, then who knows it could be anything.