Apparently Google decided they were special, and requires every relying party to implement custom crap for detecting gmail.com email addresses.

They claim that once it's stable they'll open it up so you'll be able to use "gmail.com" as your identifier in any RP:

http://openid.net/pipermail/general/2008-October/006169.html

Embrace: kinda

Extend: you betcha

Extinguish: ...