An Apple patent (number 20120278863) has appeared at the U.S. Patent & Trademark Office for ad hoc account creation. It provides a mechanism that allows a user to easily configure a rules engine to apply rules to decide which requests for access to a user's computer resources are to be granted and which are denied.
Per the patent, a trusted token, such as a certificate of identity issued by a trusted third party authority that verifies identities of computer users, is included in a calling card object provided by the requesting user to the (server) computer that controls the resources desired by the requester. Additional conditions for access may be specified as desired by the user of the server computer.
Here's Apple's background on the invention: "Proliferation of portable computers and computer networks has drastically widened the scope of computer usage for the average computer user. Access to computer networks is now offered in such public venues as coffee...| Read more »