I wonder if Yahoo's offering will be sip based or some home-grown protocol like Skype.
This may be either a big hint or a bit of misdirection. There are enough sip servers listed to make things interesting. Looks like they are expecting a large crowd or want us to think they are expecting a very crowd.
$ dig sip.yahoo.com ... sip.yahoo.com. 1800 IN CNAME sip-re2-2.voice.yahoo.com. sip-re2-2.voice.yahoo.com. 1800 IN A 68.142.233.156 sip-re2-2.voice.yahoo.com. 1800 IN A 68.142.233.157 sip-re2-2.voice.yahoo.com. 1800 IN A 68.142.233.158 sip-re2-2.voice.yahoo.com. 1800 IN A 68.142.233.159 sip-re2-2.voice.yahoo.com. 1800 IN A 68.142.233.160 sip-re2-2.voice.yahoo.com. 1800 IN A 68.142.233.161 sip-re2-2.voice.yahoo.com. 1800 IN A 68.142.233.162 sip-re2-2.voice.yahoo.com. 1800 IN A 68.142.233.163 sip-re2-2.voice.yahoo.com. 1800 IN A 68.142.233.164 sip-re2-2.voice.yahoo.com. 1800 IN A 68.142.233.165
-wolfgang