This is what I see when I tried to submit another sitemap to Yahoo! Site Explorer today.

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!. Site Explorer services will not be available from November 21, 2011.

This means, bye bye Yahoo! Site Explorer.
Well, good news and bad news. Good news is, I don’t have to worry about submitting new site to Yahoo! already, just need to submit to Bing! will do. Less one thing to worry about.
Bad news? Nah.. I don’t care.. these 2 search engines are least to care about.. submitting to them are only to catch those leftover.

Reference:
Reminder: Site Explorer
Merging with Bing