Synchronising advanced data structures with a backend sucks. Especially when the sync is both ways; user updates to server, server updates to user. Couchbase for Mobile attempts to solve this issue by bringing CouchDb and it's spiffy replication to Android.
Getting started with the development of location-based services on Android is relatively easy thanks to the well documented location API. However, getting more serious shows there is still much uncharted territory. One such area concerns the accuracy of real-life location data, which I have recently taken a close look at. In short, location data can be far more accurate than Google's conservative estimates - at least with the phone that I used.