If you are trying to ping the Guest OS and you are using NAT for networking, it might not always work. It was a limitation of ICMP protocol traffic in NAT mode for VirtualBox.
NAT in VirtualBox does have these limitations (per the User Manual):
There are four limitations of NAT mode
which users should be aware of:
ICMP protocol limitations: Some
frequently used network debugging
tools (e.g. ping or tracerouting) rely
on the ICMP protocol for
sending/receiving messages. While ICMP
support has been improved with
VirtualBox 2.1 (ping should now work),
some other tools may not work
Receiving of UDP broadcasts is not
reliable: The guest does not reliably
receive broadcasts, since, in order to
save resources, it only listens for a
certain amount of time after the guest
has sent UDP data on a particular
port. As a consequence, NetBios name
resolution based on broadcasts does
not always work (but WINS always
works). As a workaround, you can use
the numeric IP of the desired server
in the \server\share notation.
Protocols such as GRE are unsupported:
Protocols other than TCP and UDP are
not supported. This means some VPN
products (e.g. PPTP from Microsoft)
cannot be used. There are other VPN
products which use simply TCP and UDP.
Forwarding host ports lower than 1024
impossible: On Unix-based hosts (e.g.
Linux, Solaris, Mac OS X) it is not
possible to bind to ports below 1024
from applications that are not run by
root. As a result, if you try to
configure such a port forwarding, the
VM will refuse to start.