How to store GPS coordinate and search places in a radius from a NoSQL DBMS (like DynamoDB)
The classic answer is to query the database for the set of points within the 100 mile BOX and then remove the subset of points that are outside the 100 mile CIRCLE in your application layer using your programming language of choice (not SQL).

I've done this with data in Amazon SimpleDB efficiently, and you could do the same in DynamoDB with a couple "index tables" one using lat and another using lon as hash keys...