lammert - 5:28 am on May 23, 2010 (gmt 0)
Tedster, you are right that for existing sites where currently only the checkout process is SSL encrypted, regaining access to the Google referrer data would mean a re-indexing of the unencrypted pages and to reuse existing link juice a 301 redirect to the SSL encrypted page versions. New sites are easier in that respect as there is no conversion to take place, only initial setup.
Let me say that I am not proud of this solution but it is currently the only feasible solution I see to receive Google's referrer information once searchers switch to https://www.google.com.
The only possible other alternative is when Google decides to provide the SERPs through https:, but serve all outgoing links through a http: based URL redirector. I am not sure how that would impact on browsers though. They still might decide to filter the referrer info due to the https: to http: transition in that process.