I've been messing around with Obsidian Portal for my current Dark Sun campaign. Seems as though it might be a nice way to keep track of all the various NPCs and locations.
The real question is if I can get my players to actually use it.
If you want your players to use it, but don't want to be tyrannical about it, the just reward them for using it. Here are a few ways I can think of off the top of my head.
Have additional background info available in there about the story that will be of help to the party later on. For example, maybe there is a haunted dungeon that you will have them going through, and incorporeal undead fill the halls. Well, ghosts, being insubstantial and having phasing, are a real pain in the donkey. But maybe there is a hymn tied to this dungeon that can be sung to make it less painful.
For we shall not fear them (hymn)
Standard action
Close burst 5
Target: All ghosts in burst
Attack: Religion check vs. Will
Hit: Target becomes substantial and loses phasing until the end of your next turn.
But the only way for your players to find this info is by reading the notes in OP.
This would probably work best if you can get one player to do so as bait, and then continue to have tidbits like this that the players can find.
Another one, is to have the player's background develop there outside of the game. This will also reward you, since you will be able to integrate their background info into the campaign, should it ever be appropriate.
Just a few ideas.