Robert Haas committed his bare bones JSON type five days ago, and three days later I committed my array_to_json and row_to_json constructors, so this is now definitely a go for 9.2. Kudos to Robert for persisting with this - I had just about given up on us getting it done for this release.
As an added bonus for those of you who are a bit impatient to use this stuff, I have back ported this to 9.1 as an extension. You can get the code from my bitbucket repository, and when I have polished it a bit more I will publish it on PGXN.
PLEASE NOTE: this should work fine if you dump and restore the database when moving to 9.2 - just suppress the extension creation. But it won't work with binary upgrade.