nopantsyet - while I agree with your basic assertion, I do not see how it is contradictory to anything else that has been posted. Still, it is good advice and, perhaps, should be explicitly mentioned.
Anyway, to the question at hand.
For basic rules/races changes, I explicitly state it from the outset. As an example, I do not have Dwarves in my homebrew. I do have two dwarf-like races, but no dwarves. I have laid out most of the differences ahead of time so that a player can get a feel for the cultural differences.
I created a Character Creation checklist that includes my point-buy, the campaign assumptions, and the big houserules - Namely that all skills are purchased the same as class skills and that characters get 4 bonus points to be used in a Profession or Craft at 1st level.
I also included a list of the books that I use for my campaign. Double-column and two pages long, I do not expect my players to have the same library that I do. However, I feel it is a good reference for those looking to expand their library.
That handles most of the "mechanical" aspects of a homebrew. The rest is all flavor and I try to find excuses to integrate flavor whenever I can. As nopantsyet has said, this should be in a relevant context so they have a reason to care and a reason to remember. Many times, I will interject these bits if it is something the character would know, even if the player does not. Other times, I will expound on things based on good knowledge skill checks. And occasionally, I will do it as basic storytelling in game. (Such as the performing Bard at the tavern the party is at.)
You should keep a few rules of thumb when you are looking at this. First of all, do not ever penalize the players for basic knowledge that their characters would have. Be sure to pause the game and explain the information before it would have an impact. Allow the player to have a few moments to consider what the societal norm is and decide if their character fits that norm, or not. Then, let the game roll so they are making educated decisions and roleplaying because of it. Second of all, the more life you breath into your world, the more real it will seem. If you have 3 people all make a Knowledge (whatever) check, do not assume they all know the same thing. Pick out 3 distinct facts and present it to the group from the perspective of each character. As well, do not assume that only one Knowledge skills is applicable. I often have multiple Knowledge checks with different DC's. As an example:
The group comes across an ruined watch tower in the wilderness. The human paladin, the elven wizard and the human druid all know *something* about the place, based on some excellent Knowledge rolls. Maybe the paladin made a Knowledge (Religion) check and knows that the tower was once home of a vile cult that was stamped out 250 years ago by his order. The cult had been abducting people from nearby villages for 2 years before Tenbar the Righteous tracked them to their lair below the tower and wiped them out. The elven wizard makes an astounding Knowledge (History) check. He sadly comments that he remembers when the the humans first built the tower 425 years ago, they contracted some of the finest elven woodcarvers, including the wizard's uncle, to work on the stairway on the inside. Sadly, humans live such short lives and now they are all gone. As an extra, I recognize that the character also has a high Knowledge (Arcana) and throw in a gimme fact. The tower was home to a cabal of wizards. 300 years ago, there was a great explosion at the top of the tower and the cabal was never seen again. The theory is that an experiment went awry, but nobody quite knows for sure. The druid makes a Knowledge (Nature) check and realizes that the hill the tower was built on is actually a faerie mound. In all likelihood, the fey folk are still angry at the intrusion into one of their sacred places.
By doing this, I give everyone a feel for how different people view the campaign world, reward skill points put into Knowledge skills, reveal different layers of history, and hopefully draw the players into the game world. The fact that I have just fleshed out a possible adventure location also has some appeal.
Admittedly, this technique works equally well for a packaged setting, but it seems more important in homebrews since you tend to work a little harder to bring the players into your world.
As for specifics, when the players ask questions, they are more likely to listen to the answers. If you have a bunch of cultures for your Elves, but nobody is playing an Elf, or even interacting with them, they won't care if you have a 20 page document detailing the cultural differences. For that matter, their characters are unlikely to even be aware of the differences. There is nothing wrong with that. If you have your documentation, you can present it when it becomes important. Just be sure you present it as a cultural fact that the characters never picked up.
I might suggest purchasing Ronin Arts' Campaign Planner on RPGNow.com. Print out the pages and fill in the blanks and the personality of your homebrew will start to flow together in a format that your players might be able to digest easily. For your detail work, put together your pamphlets and hand-outs and hold on to them until they become relevant and important to the players.
