I believe ping is disabled at the incoming firewall to keep script
kiddies from doing ping sweeps across an entire subnet, waking up
devices that would otherwise remain dormant and causing those devices
to use precious RF resources. Try something other than ping and see if
you get different results.

You might have been able to do that with Cingular in the past, but no more.

From my are in north Texas, a ping to my cellphone dies out in Cingular's
Atlanta center. We thought they might be simply blocking a ping request,
but it goes further than that. We tried using UltraVNC with a port number
that instant messenger programs are using (since they work, we know those
ports are passable), but it still didn't work. We suspect there is likely
port sniffing going on to prevent this. This is current information as of
this fall.

I never could get Sprint to work, but that is old information over a year
old. Very likely they were and still are doing the same thing.

Carriers appear to find good money in setting you up with a public IP
address. Several thousand dollars to set you up and several thousand
dollars a month for a large business enterprise.

BTW, just tested my Cingular connection with whatismyip just to make sure
my experience is current. Nope, it can't be pinged, nor can I see a web
server being hosted on my laptop.

just curious - exactly what do you select at  - www.whatsmyip.com
as it appears to be a portal site.