This is not the first time I've heard of this strategy, and it seems to be quite profitable: "hook" a large number of users with a no-strings-attached free app or web app and get their contact info. Then create an app in the same niche, but charge for it, and email blast them all on launch day.

The last time I heard of someone doing this, they did it with a game. They released an ad-free, excellent game for free and popped up a dialog after a few starts saying "Enter your email here if you want to be notified when I release <App Name> 2!". After a year or so, he had tens of thousands of email addresses. Then he released a $2 sequel and did an "email blast" on launch day. Similarly, he shot up the charts very quickly.