Just to tie things up for FlightDeckDave...
Separation as provided by ATC to IFR aircraft in contolled airspace, and to IFR aircraft recieving a radar advisory service outside controlled airspace, is not always needed. Aircraft can fly in formation, for example, with the agreement of both (or more) pilots. Even when in controlled airspace there are circumstances when IFR aircraft do not need standard separation, for example when an aerodrome (tower) controller uses "reduced separation in the vicinity of the aerodrome" or when an aircraft is on a visual approach following traffic which the pilot has in sight.
Outside controlled airspace, VFR aircraft and IFR aircraft not receiving an ATC service simply have to miss each other.