My server is working fine, but I can't ping it

I noticed earlier, one (of many) of our EC2 instances doesn't respond to ping requests. Everything else runs fine, SSH, HTTP, FTP, Database all working perfectly, but ping fails.

This instance is based on an image we use for about 40 nodes on EC2, and I don't remember ever having this issue before. I noticed because our main 'is it up' check for each server in NAGIOS uses Ping so I noticed it.

Functionally, it's not a problem (Just started another instance and that one worked fine), but for my education (and just because I was interested), why won't ping work whenever other services will?

Sam-Rudges-MacBook-Pro:~ sam$ curl -i http://50.19.x.x/
HTTP/1.1 302 Found
Date: Tue, 14 Jun 2011 16:38:36 GMT
Content-Type: text/html; charset=UTF-8
Connection: keep-alive
Content-Length: 0
Location: /dash
Server: TornadoServer/1.2.1

Sam-Rudges-MacBook-Pro:~ sam$ ping 50.19.x.x
PING 50.19.x.x (50.19.x.x): 56 data bytes
Request timeout for icmp_seq 0
Request timeout for icmp_seq 1
Request timeout for icmp_seq 2
Request timeout for icmp_seq 3
Request timeout for icmp_seq 4
^C
--- 50.19.x.x ping statistics ---
6 packets transmitted, 0 packets received, 100.0% packet loss

(Blanked out the IP addresses, but they're the same)

From: http://aws.amazon.com/articles/1145?_encoding=UTF8&jiveRedirect=1#18

Why can't I ping my instance?

Ping uses ICMP ECHO, which by default is blocked by your firewall. You'll need to grant ICMP access to your instances by updating the firewall restrictions that are tied to your security group.

ec2-authorize default -P icmp -t -1:-1 -s 0.0.0.0/0