What's a good, cheap registrar that supports IPv6 glue?
GoDaddy support .com and .net, not sure about the others.
gkg.net also support glue records apparently, although not used them.
There's a nice list for all others here.
According to the link posted by Adam, the .us zone doesn't support IPv6 glue at all.
We use opensrs.net for our domains and our customer domains. They will manually add IPv6 glue if you ask and they are currently working on rolling out new backend code to support this in their reseller/end user system.