If typing the IP address in directly doesn't take you there, I would think it's a network problem not related to DNS.
Yahoo is pingable. What happens when you ping it? Or /usr/sbin/traceroute it?
[reno4gov@web foo]# /usr/sbin/traceroute yahoo.com
traceroute: Warning: yahoo.com has multiple addresses; using 216.115.108.243
traceroute to yahoo.com (216.115.108.243), 30 hops max, 38 byte packets
1 gateway (209.233.238.254) 15.017 ms 16.224 ms 14.435 ms
2 core3-g2-0.snfc21.pbi.net (206.171.134.130) 15.762 ms 15.504 ms 15.195 ms
3 edge2-g1-0.snfc21.pbi.net (209.232.130.3) 14.844 ms 15.410 ms 14.435 ms
4 bb1-pos3-0-oc12.sntc01.pbi.net (63.203.35.2) 16.315 ms 18.188 ms 18.126 ms
5 bb1-p6-0.pxpaca.sbcglobal.net (64.161.1.18) 18.340 ms 16.559 ms 20.359 ms
6 * * *
7 ge-3-3-0.msr1.pao.yahoo.com (216.115.101.42) 16.334 ms ge-1-3-0.msr1.pao.yahoo.com (216.115.100.150) 17.053 ms ge-1-2-0.msr2.pao.yahoo.com (216.115.100.154) 17.598 ms
8 vlan28.bas1-m.snv.yahoo.com (216.115.100.122) 19.558 ms 18.512 ms vl20.bas1.snv.yahoo.com (216.115.100.225) 19.253 ms
9 img3.yahoo.com (216.115.108.243) 18.585 ms 18.093 ms 19.415 ms
[reno4gov@web foo]#
