Short answer:
mir:~ itorres$ ndp -an
Neighbor Linklayer Address Netif Expire St Flgs Prbs
::1 (incomplete) lo0 permanent R
2001:470:95e5:1:3583:ead0:514d:d459 e8:6:88:ca:fd:7c en0 permanent R
2001:470:95e5:1:ea06:88ff:feca:fd7c e8:6:88:ca:fd:7c en0 permanent R
...
Longer answer:
You're in UNIX and you have two great commands: man (system manual) and apropos (lookup concepts on manual)
Hence:
mir:~ itorres$ apropos ipv6
Net::CIDR::Lite(3pm) - Perl extension for merging IPv4 or IPv6 CIDR addresses
Net::IP(3pm) - Perl extension for manipulating IPv4/IPv6 addresses
NetAddr::IP(3pm) - Manages IPv4 and IPv6 addresses and subnets
NetAddr::IP::Lite(3pm) - Manages IPv4 and IPv6 addresses and subnets
faith(4) - IPv6-to-IPv4 TCP relay capturing interface
icmp6(4) - Internet Control Message Protocol for IPv6
inet6_opt_init(3), inet6_opt_append(3), inet6_opt_finish(3), inet6_opt_set_val(3), inet6_opt_next(3), inet6_opt_find(3), inet6_opt_get_val(3) - IPv6 Hop-by-Hop and Destination Options manipulation
inet6_option_space(3), inet6_option_init(3), inet6_option_append(3), inet6_option_alloc(3 ), inet6_option_next(3), inet6_option_find(3) - IPv6 Hop-by-Hop and Destination Options manipulation
inet6_rth_space(3), inet6_rth_init(3), inet6_rth_add(3), inet6_rth_reverse(3), inet6_rth_segments(3), inet6_rth_getaddr(3) - IPv6 Routing Header Options manipulation
inet6_rthdr_space(3), inet6_rthdr_init(3), inet6_rthdr_add(3), inet6_rthdr_lasthop(3), inet6_rthdr_reverse(3), inet6_rthdr_segments(3), inet6_rthdr_getaddr(3), inet6_rthdr_getflags(3) - IPv6 Routing Header Options manipulation
ip6(4) - Internet Protocol version 6 (IPv6) network layer
ip6(8) - Enable or disable IPv6 on active interfaces
ip6config(8) - Configure IPv6 and 6to4 IPv6 tunnelling
ip6fw(8) - controlling utility for IPv6 firewall (DEPRECATED)
ndp(8) - control/diagnose IPv6 neighbor discovery protocol
tcllib_ip(n) - IPv4 and IPv6 address manipulation
traceroute6(8) - print the route IPv6 packets will take to a network node
To get more detail about ndp:
mir:~ itorres$ man ndp
Those packets are either blocked or corrupted by a network appliance or OS X itself (pfctl).
If you are certain that Stealth Mode is disabled I would launch a packet capture with Wireshark and see if you get anything back from your ping6.
If you have an other computer I would try to ping6 :
Mac WiFi <=> router <=> PC Ethernet
Mac WiFi <=> router <=> PC WiFi
PC WiFi <=> router <=> Mac WiFi
PC WiFi <=> router <=> Mac Ethernet
Mac Ethernet <=> router <=> PC Ethernet
It would help you to figure is Mac is not able to craft/receive ping6 packets over WiFi, or if the issue is located to the router.
You can also check the router firewall logs to see wether the firewall block those probes.
Best Answer
I assume you're talking about on your Mac since you don't specify. Yes you can do this but in all likelihood, you won't see much other than your internet connection probably won't work. Most ISPs (Internet Service Providers) are not yet ready to handle IPv6 traffic and many won't be right up to IPv6 day on June 8th 2011. Many won't be ready beyond then either. But it won't do any harm to try it out if that's what you're worried about. As an aside, the last batches of IPv4 addresses were distributed recently.
To turn IPv4 off go to System Preferences > Network > Advanced > TCPIP
Then on the drop-down list next to 'Configure IPv4' select the 'Off' Option. Then beneath that, you can set the 'Configure IPv6' drop down menu to whatever you want.