Why can't I ping a subdomain I've just created?

You need to wait a bit before all DNS entries are updated. I experienced it with updating subdomain names that it lasts at least 1 or 2 hours.

OK. I'll re-check after 1.5-2 hours. Thank you very much for your reply.

Still no resolution to IP address for emre.yafz.org, is this normal? I mean I'm connected to my linode server's shell and running the ping from there, I expect that adding a DNS A record such as emre to yafz.org to get emre.yafz.org would let me ping emre.yafz.org succesfully (from my linode server) in shorter time (but maybe then, I'm totally mistaken).

I've never had delays that long, but it is resolving for me:

→ ping emre.yafz.org
PING emre.yafz.org (109.74.204.147): 56 data bytes
64 bytes from 109.74.204.147: icmp_seq=0 ttl=52 time=120.824 ms
64 bytes from 109.74.204.147: icmp_seq=1 ttl=52 time=121.582 ms
64 bytes from 109.74.204.147: icmp_seq=2 ttl=52 time=120.739 ms

→ dig a emre.yafz.org

; <<>> DiG 9.4.3-P3 <<>> a emre.yafz.org
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 27854
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;emre.yafz.org.                 IN      A

;; ANSWER SECTION:
emre.yafz.org.          86269   IN      A       109.74.204.147

;; Query time: 34 msec
;; SERVER: 192.168.1.254#53(192.168.1.254)
;; WHEN: Tue Apr 13 10:10:49 2010
;; MSG SIZE  rcvd: 47

It's resolving fine from here too:

rtucker@arrogant-bastard:~$ dig emre.yafz.org

; <<>> DiG 9.6.1-P2 <<>> emre.yafz.org
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 11069
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;emre.yafz.org.            IN  A

;; ANSWER SECTION:
emre.yafz.org.        86400   IN  A   109.74.204.147

;; Query time: 115 msec
;; SERVER: 208.67.222.222#53(208.67.222.222)
;; WHEN: Tue Apr 13 12:54:38 2010
;; MSG SIZE  rcvd: 47

I note that the negative caching TTL is 86400 seconds for yafz.org, which means that your local nameserver is permitted to cache the fact that emre.yafz.org doesn't exist for up to 24 hours. Odds are good you tried checking it before it went live on the nameservers, and thus, you're caching.

(The last number in the SOA:

rtucker@arrogant-bastard:~$ dig yafz.org SOA +short
ns1.linode.com. emre.sevinc.gmail.com. 2010041337 14400 14400 1209600 86400

… is the negative caching TTL.)

Quick solution: try again tomorrow :-) -rt

Thank you very much for detailed answers and output. It is working for me now, too.