Keeping SESSION alive between HTTP and HTTPS? Or even across sub-domains?

Hello, all,

I've been searching Google for this, and I'm finding a lot of information about ColdFusion session variables; but I'm not finding anything (so far) on keeping session variables alive between HTTP and HTTPS, or across sub-domains (ie, from www.domain.com to img.domain.com to vote.domain.com, etc.)

Is there a tutorial or KB on this subject?

V/r,

^_^