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.