Important API Updates and Changes · YDN Blog
Especially the section about Site Explorer API being closed down!
MANY of the tools we use - Market Samurai, various browser toolbars etc. - use Site Explorer heavily for their information. Especially, SE is the most reliable way to see incoming backlinks, vital for niche research.
What's not clear from the article is whether there will be some service to replace it, in cooperation with Bing. Hopefully something even bigger and better will come along, but the news is a little worrying...