1. A method for determining differential availability, the method comprising: identifying a current context of a first entity; and determining a current availability value of said first entity with respect to a second entity in accordance with an availability function, wherein said availability function is adaptive in accordance with an adaptation process to at least one past actual availability value of said first entity with respect to said second entity in a previous context at least partially related to said current context, and wherein said availability function is operative to determine said current availability value as a function of said current context.