Here’s a thought: a scalable memcached service. So what you provide is a memcached pool. Charge by memory – it doesn’t matter how many servers this runs on. x4/MB of memcached. Then, I, as a user, just have to buy some, and call it in my code. And I can easily get more. Nice & sweet, especially because shared hosts and such often can’t easily install memcached.
I know, probably a bad idea.