We developed http://membean.com that utilizes some of this technology and we've been able to make this a successful business albeit in a restricted domain. But as others have pointed out .. it's hard to sell this. We've managed it because of stellar content built around our engine: http://membean.com/exemplars. Without excellent content it would have been hard to get this off the ground despite us able to show that we outperform existing SRS tools.
Most recently Smart.fm tried this and failed.
I don't think it occurs to many people to seek out and use memory tools. However, there are groups of people who do have a very specific need for these tools and actively seek them out. I'm thinking of people like medical students, language enthusiasts, etc... Anyone who has an immediate need to learn and recall large amounts of information.
By intentionally limiting the scope of what you offer, you can better target prospects and tailor your offer to fit their needs like a glove.