The SRD doesn't contain any PI. That's why I could, and others *have* taken the entire thing, spruce it up a bit, and released it as a d20 product. It's perfectly, and legally copyable.
There are really three things in a book:
OGC
PI
other
OGC is anything that is derived from the SRD, or other OGC, which in turn is, by definition, derived from the SRD.
PI is anything that you, as the author, declare that is forbidden to be copied. It is generally a proper name, or a story, or something that is unique to your product, that makes your product special. PI can not be copied. Therefore, if I write a spell, and give it a name "die_kluge's wonderment" and make the name PI, no one can really use my spell. But, the spell mechanics (everything but the name) have to remain as OGC, because they are specifically derived from the SRD. This, is called crippled OGC, because while it is perfectly legal, publishers hate it because they either have to get permission to use the name, or they can't use it at all.
"other" is everything else. It is up to the publisher to decide on how to classify it. Take a module for example. A module might have a dungeon and monsters, and treasures. There might be some new crunchy bits for PrCs, and other nifty things in there as well. All that is derivitive of the SRD. It all has to be defined as OGC. The mechanics, that is. All of the names of all the monsters, and all the spells, and the names of the magic items - I can declare those as PI, thus rendering all my OGC "crippled". Or not. It's my choice.
But there is more to a module than that - there is the story, the plot, the sequence of events. This is neither PI, nor OGC, since it's not entirely proper nouns, and it's not derivitive of anything, but a sick, twisted imagination. Now, I have a choice. I can label it all as PI or I can choose to label it all as OGC. The choice is mine. I'm not dictated one way or the other. Most label it as PI, since you don't want someone taking your work in its entirety and re-publishing it (as if that would actually happen).
Nowadays, most publishers have gone to one extreme or the other. Some are of the 100% OGC camp. I love these folks. Essentially, and this is personal opinion, these publishers have decided that no one is really making any money at this whole d20 thing anyway, so they've decided that no one out there really is evil enough to take an entire work and re-publish it (no money to be gained by doing that, and a whole lot to lose, reputation-wise), so it's easier to just declare it as OGC. This happens quite often, unless you're writing in a heavily-branded campaign setting (Scarred Lands, for example).
Others choose to just be intentionally vague with their OGC designations, preferring to have the publishers come to them directly to ask. This, too, is a somewhat lazy approach to it, since it's easier to just say "ah, come ask us", than it is to specifically identify what is, and what is not OGC. It's cheaper, for one thing.
There are a few others still, that intentionally, and methodically go through and figure out which parts are, and which parts are not PI or OGC, and designate them as such, believeing (probably erroneously, and wishfully) that that single spell name, or magic item name is somehow going to be a big cash cow for them in the future (see: Mordenkainen) and so they protect their stuff like stingy, paranoid children.
That's the long and the short of it.
And before you ask, yes, I've published under the d20 license.