On Mon, 2012-10-01 at 09:36 +0100, Chris Clayton wrote:

     0 ICMP messages received
     0 input ICMP message failed.
     ICMP input histogram:
     0 ICMP messages sent
     0 ICMP messages failed
     ICMP output histogram:

After:

$ netstat -s
Icmp:
     4 ICMP messages received
     4 input ICMP message failed.
     ICMP input histogram:
         echo replies: 4

So icmp replies come back and are delivered to host instead of being
forwarded.

I wonder if MASQUERADE broke...

Could you send

iptables -t -nat -nvL
conntrack -L   # while ping is running from guest