I would pay DropBox premium for this feature.
That's not how it works.
People didn't leave Hotmail because the "educated" left, they left it because Google gave several GBs for free, the interface was simpler, the search better and MS was out of fashion.
If you're not a large market you don't get a service, or you only get niche vendors to cater to you. You can bypass this by setting trends for the "uneducated" (whatever that means), else we will all be using Lisp Machines or Smalltalk environments.
Privacy is becoming a large problem in the internet and encryption will likely be part of the solution. Without encryption, the ownership of data is on the service provider instead of the person.
Privacy is a feature just like free storage. One day, privacy can be available to the masses just like storage is today. (also think back how many people actually wanted or needed multiple gb of free storage for their emails until one was provided by a service like gmail)
The BIGGEST deepest pockets would pay handsomely for this.
That is to say, you can't be sure. However, Dropbox is a company in good legal standing, and they have a lot to lose if they offered client side encryption and then leaked the passphrase.
Spideroak (definitely) and Backblaze (I think?) already have client software which offers client side encryption. Whether you trust them is up to you.
The product is in beta right now but I'd love some more people to try it out. If you sign up at http://safeboxapp.com, you'll get a download link to try it out.
How does it compare to BoxCryptor where you can also access your files using EncFS?
Please tell me you are sending out invites today.