I finally got myself internet data on my plan, and signed up for the beta shortly after (within 24 hours). Today I spent all day working on it.

I noticed, for starters, that the ccinternet2.t-mobile.com APN didn't work for me, and that my e-mail confirming my T-Mobile IPv6 beta activation contained a different APN address that did work.

Is T-Mobile just giving different APNs out to customers on the beta in different regions or something?

At any rate, I found that following this: http://n900-ipv6.garage.maemo.org/

...worked fine for me, although everything worked for me with this as my /etc/resolv.conf file:
I do not have an IPv6 only WiFi network to see if not having a specific address in resolv.conf for IPv6 instead of ::1 (which is localhost, like 127.0.0.1) breaks anything, but I think it won't.

I did however notice that having the IPv4 entry first in resolv.conf broke my ability to get IPv6 DNS resolution, whereas both IPv4/6 work if I have the IPv6 locahost entry first.

*Shrug* The whole resolv.conf thing is rather confusing to me, but I just thought I should point out that you don't have to use "nameserver 2001:470:b:c8::1" as suggested by the link about, or "nameserver [T-Mobile's nameserver]" in your /etc/resolv.conf file itself (especially since T-Mobile's DNS nameserver address can be added by the script that adds the gprs1 interface from the above link anyway - I'm also not sure it'd be ideal to be using the T-Mobile DNS server for, say, WiFi browsing on non-T-Mobile IPv6 networks). Since the default in that file is "nameserver 127.0.0.1", it seems to me that the 'proper' way is to use localhost address in IPv6 in that file as well for the /etc/resolv.conf nameserver entry, and leave the actual DNS resolution to 'upper' settings/services. I presume localhost addresses for nameservers lead to resolution using dnsmasq (spelling?) on first try, so it seems if you need special provisions on the N900 for IPv6-only networks, there's better ways to do it then set an external nameserver in the /etc/resolv.conf.