Just a suggestion that cost me a couple days trying to get Explorer:
I'm running tunnelless because I'm using the 6to4 that is "just there". Windows' DNS resolver when confronted with AAAA and A records will always return the A in this configuration. Firefox relies on Windows' resolver, whereas Opera apparently uses its own resolver. The latest Opera defaults to AAAA, and allowed me to pass Explorer.
If you're using 6to4 on Windows and having trouble convincing ipv6.he.net that you are, indeed, using IPv6...try Opera.