It seems to me that there are two approaches to reflecting the importance of knowledge within a game. I'll label them "meta rewards" and "in-game rewards", and deal with them in reverse order.
In-game rewards are what players get when they use knowledge to unlock more useful knowledge, or to otherwise gain an advantage within the game. Example 1: players in a CP2020 game meeting an important NPC at a party, getting to know him, offering to do something for him. They now have increased their pool of knowledge (they know another NPC) and this might be used to further their aims later on (the NPC is important, has connections, can do things for them or get them connected to more knowledge through contacts, etc.). Example 2: players come across a band of goblins in a D&D game, talk to them, hear a rumour from them of a dangerous sorcerer in the local area. They have increased their pool of knowledge and now have access either to an important helpful NPC or somebody they might want to rob. Example 3: players break into an archive, discover a treasure map. They have now found a way to get both money and/or XP.
The aim of the in-game reward is to, basically, positively reinforce active and intelligent player involvement in the game setting. The reward itself is slightly tenuously linked to the activity (it is not immediate, and may come in the form of gold or XP months of game-time later), but the idea is that knowledge about the setting has tangible in-game benefits.
A meta reward would be giving actual XP or equivalent for discovering new information. Example 1: players in a D&D game are explorers. As they uncover more hexes or geographical/cultural information, they get XP rewards. Example 2: players in a Call of Cthulhu-type investigative game are given XP rewards or equivalent for finding new, hidden knowledge about the Other Gods they discover in an ancient tomb. Example 3: players in an Unknown Armies-type urban fantasy game are given XP rewards or equivalent for discovering that Tom Cruise actually is a thousand-year old Prolonger.
The aim of the meta reward is, obviously, to reward the seeking of knowledge for its own sake, as an intrinsic element of the game: the game is about finding things out.
I basically only ever use in-game rewards, because that's the style of game I generally run, but I certainly think that XP for exploration is something that I would like to consider for a D&D game, and that XP for revealing information about Horrible Ancient Stuff would be a great mechanism for a D&D/Call of Cthulhu mashup.
There is, of course, a point at which in-game rewards and meta rewards tie together, in that an in-game rewards (find out about an NPC and carrying out a mission for him, in return for which he gives you a map, which eventually leads you to treasure) morphs into a meta reward (you get XP for the treasure, which means levels). This is one of the many ways in which the creators of D&D sort of awkwardly stumbled across pure game design genius: D&D rewards active player involvement with all elements of the setting both within the game (knowledge = opportunities to get more treasure/power), in the meta game (opportunities to get more treasure/power = XP/levels) and back again (more XP/levels = more power and advancement within the game setting). It's a virtuous circle which few if any games can match when done right.