LinuxQuestions.org - Some domains can be resolved but others can't

I have a ubuntu installed in vmware. I don't remember what I did before the problem occurred. In ubuntu, some domains can be resolved such as www.google.com but others just like www.ubuntu.com can not be resolved.
I can't figure out why this happen.
Here's output of dig in my ubuntu:
Sames ok for www.google.com
roy@RoyDesktop:~$ dig www.google.com

; <<>> DiG 9.6.1-P2 <<>> www.google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35682
;; flags: qr rd ra; QUERY: 1, ANSWER: 7, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;www.google.com. IN A

;; ANSWER SECTION:
www.google.com. 5 IN CNAME www.l.google.com.
www.l.google.com. 5 IN A 74.125.71.106
www.l.google.com. 5 IN A 74.125.71.147
www.l.google.com. 5 IN A 74.125.71.99
www.l.google.com. 5 IN A 74.125.71.103
www.l.google.com. 5 IN A 74.125.71.104
www.l.google.com. 5 IN A 74.125.71.105

;; Query time: 46 msec
;; SERVER: 192.168.207.2#53(192.168.207.2)
;; WHEN: Thu Apr 29 10:40:49 2010
;; MSG SIZE rcvd: 148

but www.ubuntu.com can't be resolved
roy@RoyDesktop:~$ dig www.ubuntu.com

; <<>> DiG 9.6.1-P2 <<>> www.ubuntu.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: REFUSED, id: 55562
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;www.ubuntu.com. IN A

;; Query time: 33 msec
;; SERVER: 192.168.207.2#53(192.168.207.2)
;; WHEN: Thu Apr 29 10:42:09 2010
;; MSG SIZE rcvd: 32

PS: in my host OS, both google.com and ubuntu.com can be resolved.
ubuntu in vmware used NAT to access network

Thanks very much for your help