Yahoo! had some availability problems this morning.

<< At its worst point, Yahoo.com's availability dropped to around 60 percent, meaning that 4 out of 10 visitors couldn't access the page, he said, citing data collected from Keynote's global network of monitoring computers, which check the availability of the world's most popular sites. >>

http://news.yahoo.com/s/pcworld/20070706/tc_pcworld/134237;_ylt=AlN373rMWQf9wxXswsrjarnMWM0F

This was covered by Netcraft.com and Internet Storm Center too:
http://news.netcraft.com/archives/2007/07/06/performance_issues_for_yahoo.html