Some people are noticing Google referrals from search strings that look like:
https://www.google.com/evaluation/search/rating/task-edit?task=XXXXXX
This comes from Google's http://eval.google.com/ which only available to those part of the "EWOQ group." SearchBistro has a ton of details with demos of it in action named Google Secret Lab, Prelude.
There is some old forum discussion on it at Search Engine Watch Forums.
When I tried accessing one of the referral strings I was prompted with a 403 forbidden code saying:
ForbiddenThe user [username]@gmail.com is not a member of EWOQ. Please contact the ratingprojects@google.com for access.
Forum discussion at Search Engine Watch Forums and Google Blogoscoped Forums.

Comments:
Matt Cutts
11/28/2006 04:02 pm
That Search Bistro speculation is also old (2005), in case you thought it was new.
Barry Schwartz
11/28/2006 04:04 pm
Yea, I know it is old stuff. But the referral links are new, new threads on them.
vicyankees
11/29/2006 02:45 am
Barry - thanks for the info. I had found the old links before posting but was curious if there were any new developments. Matt - Any other details?
Brian Ussery
12/04/2006 02:57 pm
http://eval.google.com/ has been open to any user for a few months.
Zebulon
03/10/2007 06:20 pm
I got this kind of link on my referral today so i really hope that's a good thing for me ^^
SEO Ranter
07/10/2007 01:45 pm
So did you contact them?
No Name
04/18/2008 01:32 am
I just got one referer from this hidden google lab. They have some interesting values in the url which is : task-edit?task=xxxxxxxxxx. It appears to me that they try to rerate their broken index entries if you score too high on a sertain keyword.