Didn't help (I'd tried it).
This solves it for MacBook Air - see above for details.
networksetup -setv6off Wi-Fi
It's important to get the capitalization correct, I suspect.
For Ethernet on Macbook Air using the USB Ethernet dongle, it's:
networksetup -setv6off USB Ethernet
Come on Apple, IPV6 is dead. Move on to ISO CLNS, please. ISO is supported by all major router vendors and IS-IS (ISO routing protocol) is used by many ISPs instead of OSPF or OSPF6 (IETF routing protocols), mainly because the ISO protocol is stable, and IS-IS is more robust than OSPF.
Second: IPv6 should be off by default. Configuring any unused protocol on by default (even appletalk, netware, etc) is a major security vulnerablity. There are several studies of using IPv6 to penetrate firewalls, leaving apple devices vulnerable to remote exploitation if IPv6 is enabled.
I have quoted a 2009 IPV6 "success story" below, that aptly sums up the irreparable flaw in IPv6. This flaw is why IPV6 hasn't caught on over the last 10 years.
"Domain names in Domain Name Servers (DNS). This is a Catch-22 question when planning to support IPv6: when to create the AAAA record in the DNS to associate the IPv6 address of a server with its domain name? The AAAA record should not be created until after all services running on a server have been IPv6-enabled. But even so, there is no “right” time. Many of the operating systems on today’s laptop and desktop computers are IPv6-enabled by default (a major exception being Microsoft Windows XP). An IPv6-enabled system may make DNS queries for IPv6 addresses and then attempt to access them, even though the network it is on supports only IPv4. Such attempts can time out for various reasons, and the system will then fall back to IPv4. Timeouts can cause user frustration or confusion. The timeouts are often blamed on IPv6 (do a web search on “ipv6 web slow”), when IPv6 may not the problem6. "
One would expect the remainder of the paragraph (and footnote 6) to try to explain why IPv6 may not be the problem, but it doesn't. Rather, it advises users to suffer the problems until everyone converts. Right; like that'll happen. IPV6 dual stacking is the problem. Dual stacking has never worked in the history of computing, going back tot he 1950's for this reason. It is rather amusing to note that this report itself starts with quotes of IPV6 "success" since 2003 and again in 2007. It seems obvious that their measure of success and our measure of success are quite different. One could go on about the many other IPV6 flaws such as protocol layering flaws in IPV6 which are implicit in the document in portions regarding MLPS, but seems unnecessary. One could go further and add the argument that IPv6 doesn't actually increase the number of IP addresses available and so it doesn't even solve a problem. So come on Apple, Let's move on!
Good luck. Otherwise, I'm very pleased so far with Lion and the Retina Macbook, and look forward to upgrading to Mountain Lion soon.