What is meant by access-controlled search? Basically I would like some of my own content to be searchable by some of my friends. This is probably something that most social sites want to add; but can't add because most of such information is in emails (or in desktop).
Why is google (through gmail) in a good position to implement this? Here is the recipe:
In essence, to me it looks like best possible UI for access controlled search based on contacts: Associate labels to contacts and mark the label with read permissions. One could even mark it as "public" and let it be searchable by google - after masking off From/To/Cc and dates.
In any case, the real problem is: "How do we label the content efficiently?" Labeling contacts is easy and done only once (such as "Family", "My School Friends", "My new startup biz contacts", "VCs".) This in turn allows emails from those contacts to have same label and this is good thing.
This might as well be a killer feature for google.