Nope. As of yesterday the site was working fine and had no troubles. But I did read somewhere that it was going to be shut down and be replaced with something else. But that hasn't happened yet.
May be you not checked it well because is still not working. The following message is appearing:- "With the completion of algorithmic transition to Bing, Yahoo! Search has merged Site Explorer into Bing Webmaster Tools. Webmasters should now be using the Bing Webmaster Tools to ensure that their websites continue to get high quality organic search traffic from Bing and Yahoo!".
Ya its merged with Bing webmaster tool and sign up is an easy process Bing will also show your back links.
It was on the cards for a while since the venture with MSN, depending on where you stand it could be a good idea, not many admins want their backlinks revealed to their potential competitor