1. A method for determining differential availability, the method comprising: a) identifying, using a context sensor, a current context of a first entity; b) determining a current availability value of said first entity with respect to a second entity in accordance with an availability function maintained for said second entity; c) notifying said second entity of said current availability value of said first entity with respect to said second entity by presenting a representation of said current availability value of said first entity with respect to said second entity via a computer-controlled output medium, wherein said current availability value of said first entity with respect to said second entity indicates a likelihood that said first entity will communicate with said second entity responsive to a communication request sent by said second entity to said first entity, wherein said availability function maintained for said second entity 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 maintained for said second entity is operative to determine said current availability value of said first entity with respect to said second entity as a function of said current context; d) determining a current availability value of said first entity with respect to a third entity in accordance with an availability function maintained for said third entity; and e) notifying said third entity of said current availability value of said first entity with respect to said third entity by presenting a representation of said current availability value of said first entity with respect to said third entity via a computer-controlled output medium, wherein said current availability value of said first entity with respect to said third entity indicates a likelihood that said first entity will communicate with said third entity responsive to a communication request sent by said third entity to said first entity, wherein said availability function maintained for said third entity is different from said availability function used to determine said current availability value of said first entity with respect to said second entity, wherein said availability function maintained for said third entity is adaptive in accordance with an adaptation process to at least one past actual availability value of said first entity with respect to said third entity in a previous context at least partially related to said current context, and wherein said availability function maintained for said third entity is operative to determine said current availability value of said first entity with respect to said third entity as a function of said current context.