I also had an idea a while back regarding rituals and skill challenges. Not in the acquisition of the rituals, but in the actual casting of them.
Many of them have associated skill checks already. Making that skill check a skill challenge could be really fun. Imagine a group of people working together to summon a demon, a task which, if they get wrong, will result in a major problem for them!
I'm not sure how I'd break down the various skill rolls, etc. But the process of summoning the demon could be an encounter in itself.