The Bing Developer team recently announced that the "Bing Box" functionality on bing.com/siteowner will be discontinued on April 4, 2011. From 4th April 2011, Websites attempting to use search box will be redirected to a 404 page. That means websites interested in receiving web or site search results from Bing, need to be switch to Bing API.
Here what they say:
We have decided to take this step for two reasons.
Second, the functionality available through bing.com/siteowner can be replicated with the Bing search API and so it feels a bit redundant.
The Bing box powered by Bing provides a fast, customizable search solution for your website or blog. Visitors to your site can search the entire Web, just your site, or any sites that you choose.