delericho
Legend
For me, the back-story sets the tone and feeling of the adventure; it inspires the DM. Perhaps there are degrees of this, and maybe a more "impressionistic" back-story is preferable to a detailed one, but the point being that there's a huge difference between the following:
A) In Room 32 there's a lich with the following stats
B) Room 32 is the crypt of the long-dead priest king Amennatep II, now a lich. Amennatep was the last ruler of the 44th Dynasty of Medalhur, called the "Cursed Dynasty" because....etc
A question with regard to B: is that information something that's going to change the way the adventure plays out, or is it just a "cool story" for the DM to read?
If it's the latter, just a "cool story" for the DM, then I think I'm inclined to agree with Lanefan - stick it in the introduction so that the DM has the tone and feeling set before he starts reading the adventure, and also so that information he may be stripping out isn't cluttering up the adventure when he's trying to use it.
(And, I think the designers then need to consider just how much of such flavour text goes in at all. It's eating up the limited word-count, so if it doesn't affect the adventure, maybe (maybe!) it shouldn't be there at all.)
However...
It has been my experience that players very often* just gloss over "boxed text"... but not just boxed text, but pretty much anything the DM tells them by way of exposition. Indeed, at their worst they can be quite blasé about pretty much anything done to set the tone, feel of an adventure, or whatever. And so, "Amennatep II" just becomes "that lich guy", and the "44th Dynasty... Cursed Dynasty..." just becomes "blah blah blah... what treasure does he have?"
* Important: "very often", not "this is universally true"!
But if they were the "Cursed Dynasty" because they angered Ra by stamping his symbol on their gold currency and then debasing that currency to near-worthlessness, then that matters - it means that that massive haul of gold coins the PCs are lugging around is probably worthless, and they should go back and get the silver they left lying. And if the curse took the form of a plague of rotting skin, then those heiroglyphs depicting a purification ritual (that the players ignored three rooms back) suddenly matter a great deal.
The problem is, though, that that does require a more in-depth backstory, and it does also mean that it's much harder to change - it's coupled to the rest of the adventure, so if you change it then some other stuff will need changed too.
I'm certainly not going to claim there's "one true way" to answer this. There is clearly a place for adventures with minimal backstory, to make adaptation easy. I think there's also a place for adventures with much more in-depth backstory that is then coupled to the adventure structure. And, indeed, a good adventure can come of either approach.