That way you don't need to bother with user registration, you don't need to worry about people trusting you with their sensitive information (signing-up, posting notes etc), people are already using Evernote in their regular workflows to take notes and add reminders, and if it becomes a success then an Evernote acquisition is an obvious play.
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Why don't you just add this scheduled email/text reminder functionality on top of Evernote using their API? http://dev.evernote.com/documentation/cloud/
That way you don't need to bother with user registration, you don't need to worry about people trusting you with their sensitive information (signing-up, posting notes etc), people are already using Evernote in their regular workflows to take notes and add reminders, and if it becomes a success then an Evernote acquisition is an obvious play.