Wikipedia has now moved to a round-robin DNS-hosted solution:
C:\Documents and Settings\Stephen Donner>nslookup en.wikipedia.org
*** Can't find server name for address 192.168.2.1: Non-existent domain
*** Default servers are not available
Server: UnKnown
Address: 192.168.2.1
Non-authoritative answer:
Name: rr.pmtpa.wikimedia.org
Addresses: 207.142.131.204, 207.142.131.205, 207.142.131.206, 207.142.131.210
207.142.131.213, 207.142.131.214, 207.142.131.235, 207.142.131.236, 20
7.142.131.245
207.142.131.246, 207.142.131.247, 207.142.131.248, 207.142.131.202, 20
7.142.131.203
Aliases: en.wikipedia.org, rr.wikimedia.org