A prime couple from Oerth moves to the Outlands and starts a family. Are their children planars or are they still primes? Does it take a generation or two to become planars (ie, their grandchildren qualify)?
Their children would have the Outlands as their home plane regardless, right?
Thanks for clarifications.
How about regarding whether they sense portals or are immune to banishment?
I always assumed that you had to either be indigenous or planetouched to avoid being banished, or at least that's how it is in my games-- I basically added a new subtype called "Indigenous", which determines whether or not the creature can be banished, and whether or not certain damaging features or traits of the plane affect them (for instance, whether or not they can contract Coldfire Ruin, a transmutational reaction that functions as a supernatural disease very similar to slimy doom, except that the subject turns into Coldfire/Coldflow)
Sensing portals... really depends on how you want to play both the fluff and mechanics. Can only native-born Sigilians sense portals? Or can the native-born inhabitants of the City of Brass do so as well?
By the rules, all primes are considered to be on their home plane everywhere for purposes of protection from evil, banishment, etc.; that's what they were talking about with that. And also by the rules, all planars can sense portals.
It doesn't quite seem like something that would occur after one generation to me, but after two or three I could see it happening.
I did another search through both the 2e and 3e campaign settings and couldn't find a clear answer. Interestingly enough, the 3e PSCS doesn't mention the prime "never extraplanar" benefit, though it does speak of the portal sensing. I'm not sure if that was an oversight or intentionally left out. I use it anyway, sense I prefer to give prime's some benefit to balance them with planars.
From what I could see, the closest answer was that it sometimes mentiones planars being planars by virtue of being "born" out on the planes. So the metaphysically simplest answer would be to say it's all about the moment of birth. But that somehow seems less satisfying. It also means that if a pregnant prime adventurer happens to give birth on another plane, the child is a planar.
From a bio-physical point of view, it would make sense that either the planar nature has to sink into their genes, ie, a couple generations at least, or they have to have bodies made of planar stuff, ie, they have to be gestated out on the planes. An alternate method might be to say that the parents have to live out on the planes for a few years (they say it takes a human about 7 years to replace every cell in their body, I believe) and then they can have planar children. But science and D&D shouldn't really be mixed that much, IMO.
So I'm not sure what I want to do. I kind of want to come up with a method that is easy to determine and makes some sort of metaphysical sense.
It's actually more than a speculative exercise, as I intend to run a long-term campaign (technically it's "Planescape Lite," or has strong Planescape elements without me wanting to officially designate it as a Planescape campaign), where primes might very well settle down, and or, have children on the planes.
If it merely took just spending a length of time on the planes to become a planar, there could be a whole class of people who could spend their time trying to skirt that delicate balance between remaining Prime and going native. I'm sure that could lead to all sorts of shenanigans.
to alter your status as a planar/prime for supernatural purposes, (notably ability to sense portals) id reccomend that a WISH Spell be neccessary.
That seems a little excessive to me, honestly, to the point that no player would actually choose to do it since there are so many more effective things a Wish could be used on. If you're going to give a player an option that they wouldn't actually take under any reasonable situation, you haven't actually given them an option at all. Plus, then it's kind of boring in-game, unless you make
I'd suggest instead that there be some sort of ritual related to a planar touchstone for conversion to planar, or something of a similar nature for conversion to prime. Something difficult and full of plotty stuff, perhaps one of the topics of an entire plot arc. But not so much the focus that it would push the spotlight onto the single character for the duration of the adventure and leave the rest of the party with no plot impetus themselves.
^Sounds about right.
Planewalker's Handbook, page 69, last line of paragraph 2: "Those halflings now live on the planes, but they're not planar; after a generation or two, though, their kids will be planar."
So it looks like it's probably a generational thing, rather than an exposure thing.
...You know, I thought there was something about Barncastle that said something like that, but I forgot about the thought before I thought to actually look it up. Thanks for finding it, Wicke. 
Think about 2nd generation immigrants. Their playmates will be planars, and if they get a public education, it will be as planars. Whether or not they're considered planars depends on the outlook of each individual-- I created a sect which is strongly anti-planar immigration, and they wouldn't consider the kids to be planars.