While the decision to keep his health private is definitely Jobs' prerogative, it would also make sense of him to take advantage of this crisis of faith to address the issue of succession, which will inevitably come up again at a later date, even if he is given a clean bill of health tomorrow.
Dr. Tantillo ('the marketing doctor') did a recent post on his branding blog (blog.marketingdoctor.tv), asserting that Jobs and Apple are two separate brands--Jobs one that is irreplaceable but that can, at this juncture, help ensure Apple's longevity--and that, with the question of Jobs' health at least being raised, this is definitely the time to do so.
Here's a link to the full post:
http
/blog.marketingdoctor.tv/2008/07/24/brand-advisory.aspx