the problem with having both a "bright line" rule and "reasonable person" analysis is they don't always mesh. One could hypothesize situations where  a passenger would feel free to leave (e.g., the car is stopped right in front of the passenger's house and the cop says "leave.") but in the vast majority of cases a person riding in a car clearly does not feel free to leave. Of course, it is more complicated than just that, because does the person not feel free to leave because of the perception  governmental compulsion or just because leaving is often not a practical alternative?

  This particular case seems an odd choice for the Court to take up the issue because it is factually multifaceted  both because of the arguably independent justification for the search (an outstanding warrant which arguably would make the search of his person a legal search incident to arrest) and because if the passenger has hidden contraband in someone else's car he probably has no standing to challenge that seizure even if his person has been seized illegally.

  It would seem to me that he can only successfully argue that anye contraband taken from his person must be suppressed and even that only if he can convince the court that what would otherwise be a legal search of his person incident to arrest is so tainted by the illegal initial stop (assuming it is held to be an illegal seizure of his person) and the lack of attenuation between the stop and the arrest requires suppression under the fruit of the poisonous tree doctrine.

Here's what I see to be the major flaw in any court's reasoning that a passenger is not seized.  If a police officer pulls over a car and the passenger (supposedly) feels free to leave, what do you think would happen if he went on his merry way?  Imagine this scenario: a police officer gets a call that a red Honda with two passengers just executed a drug deal and is driving south on the freeway.  The officer spots a red Honda with two passengers heading south on the freeway.  The car is stopped, and the passenger says, "I'm not seized, just the driver.  See ya!"  I would think an officer might have new grounds for a Terry frisk of the passenger in that situation.  So even if you're technically in the clear at the time of the stop, once you TRY to leave, you may arouse new suspicions in the officer that rise to the level of Terry.

  All people arrested are detained but not all people detained are arrestd. Arrest refers to a formal legal event while detention refers to a condition.

   If you are not free to leave, or a reasonable person in YOUR position would not feel free to leave regardless of the subjective belief of the officer as to your freedom,  you are "detained."

  For most 4th amendment purposes "detention is the key factor not arrest and this is also true of 5th amendement (right against self incrimination) and 6th (right to counsel) issues.

  Once you are in detention, for example you must be given Miranda warnings before being interrogated or your statements will be inadmissible against you  in court. The key issue is whether the police conduct a "custodial interrogation" and an "arrest" is not necessary.

  Police may (and very often do) detaine people without making an arrest. To be lawful such detentions must be no longer than necessary to conduct a reasonable investigation where the police have a reasonably articulable suspiciion of criminal activity. Police may conduct a cursory pat down search at this point for weapons to ensure safety of officers and bystanders. (Often known as a Terry stop or stop and frisk).

 Then (you can see the huge potential for abuse here -- and abuse that is often very hard to establish in court) if the patdown reveals something the police consider to feel  suspicious -- like a weapon the police are allowed to conduct a further search on the theory the patdown has now established probable cause for a more thorough search. If the patdown merely reveals something suspicious but doesn't seem to be a weapon the officer can either seek a warrant claiming probable cause based on the suspicious concealed object and other circumstances-- or more commmonly simply ask the person to empty his pockest or to submit to a consent search (you'd be amazed at the number of people who do).

  A person is detained from the intitiation of the stop and might be further detained after the patdown based on the alleged probable cause but may not be formally arrested for a considerable period of time.