I'm seeing a similar problem since about 48 hours ago. I'm not using comcast and use the tserv15 endpoint. Definitely something up. I emailed support but haven't heard back yet. All my queries to he.net dns result in SERVFAIL. I have switched to my own DNS resolver but now don't get the ipv6 google services. Using he.net's ipv6 dns doesn't help in my case.
$ dig @74.82.42.42 AAAA www.google.com
; <<>> DiG 9.8.0-P1 <<>> @74.82.42.42 AAAA www.google.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 35393
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;www.google.com. IN AAAA
;; Query time: 260 msec
;; SERVER: 74.82.42.42#53(74.82.42.42)
;; WHEN: Sun May 15 19:03:15 2011
;; MSG SIZE rcvd: 32